This text is a sponsored by Sentry.io
Google Lighthouse has been some of the efficient methods to gamify and promote internet web page efficiency amongst builders. Utilizing Lighthouse, we will assess internet pages based mostly on general efficiency, accessibility, web optimization, and what Google considers “finest practices”, all with the press of a button.
We’d use these assessments to guage out-of-the-box efficiency for front-end frameworks or to have a good time efficiency enhancements gained by some diligent refactoring. And you recognize you like sharing screenshots of your excellent Lighthouse scores on social media. It’s a well-deserved badge of honor worthy of a confetti celebration.
Simply the truth that Lighthouse will get builders like us speaking about efficiency is a win. However, while I don’t need to be a celebration pooper, the reality is that internet efficiency is way extra nuanced than this. On this article, we’ll study how Google Lighthouse calculates its efficiency scores, and, utilizing this info, we’ll try to “hack” these scores in our favor, all within the identify of enjoyable and science — as a result of ultimately, Lighthouse is just , however tough information for debugging efficiency. We’ll have some enjoyable with it and see to what extent we will “trick” Lighthouse into handing out higher scores than we might deserve.
However first, let’s discuss information.
Area Information Is Essential
Native efficiency testing is an effective way to know in case your web site efficiency is trending in the appropriate route, but it surely received’t paint a full image of actuality. The World Vast Net is the Wild West, and collectively, we’ve virtually actually misplaced observe of the number of system varieties, web connection speeds, display screen sizes, browsers, and browser variations that individuals are utilizing to entry web sites — all of which may have an effect on web page efficiency and person expertise.
Area information — and many it — collected by an utility efficiency monitoring software like Sentry from actual folks utilizing your web site on their units offers you a much more correct report of your web site efficiency than your lab information collected from a small pattern measurement utilizing a high-spec super-powered dev machine below a set of managed circumstances. Philip Walton reported in 2021 that “virtually half of all pages that scored 100 on Lighthouse didn’t meet the really helpful Core Net Vitals thresholds” based mostly on information from the HTTP Archive.
Net efficiency is greater than a single core internet very important metric or Lighthouse efficiency rating. What we’re speaking about goes manner past the kind of uncooked information we’re working with.
Net Efficiency Is Extra Than Numbers
Velocity is usually the very first thing that comes up when speaking about internet efficiency — simply how lengthy does a web page take to load? This isn’t the worst factor to measure, however we should keep in mind that velocity might be influenced closely by enterprise KPIs and gross sales targets. Google launched a report in 2018 suggesting that the chance of bounces will increase by 32% if the web page load time reaches increased than three seconds, and soars to 123% if the web page load time reaches 10 seconds. So, we should conclude that changing extra gross sales requires decreasing bounce charges. And to scale back bounce charges, we should make our pages load quicker.
However what does “load quicker” even imply? Sooner or later, we’re bodily incapable of constructing an online web page load any quicker. People — and the servers that join them — are unfold across the globe, and fashionable web infrastructure can solely ship so many bytes at a time.
The underside line is that web page load shouldn’t be a single second in time. In an article titled “What’s velocity?” Google explains {that a} web page load occasion is:
[…] “an expertise that no single metric can totally seize. There are a number of moments in the course of the load expertise that may have an effect on whether or not a person perceives it as ‘quick’, and when you simply focus solely on one, you would possibly miss unhealthy experiences that occur throughout the remainder of the time.”The important thing phrase right here is expertise. Actual internet efficiency is much less about numbers and velocity than it’s about how we expertise web page load and web page usability as customers. And this segues properly right into a dialogue of how Google Lighthouse calculates efficiency scores. (It’s a lot much less about pure velocity than you would possibly assume.)
How Google Lighthouse Efficiency Scores Are Calculated
The Google Lighthouse efficiency rating is calculated utilizing a weighted mixture of scores based mostly on core internet very important metrics (i.e., First Contentful Paint (FCP), Largest Contentful Paint (LCP), Cumulative Structure Shift (CLS)) and different speed-related metrics (i.e., Velocity Index (SI) and Whole Blocking Time (TBT)) which can be observable all through the web page load timeline.
That is how the metrics are weighted within the general rating:
Metric
Weighting (%)
Whole Blocking Time
30
Cumulative Structure Shift
25
Largest Contentful Paint
25
First Contentful Paint
10
Velocity Index
10
The weighting assigned to every rating offers us perception into how Google prioritizes the completely different constructing blocks of person expertise:
1. A Net Web page Ought to Reply to Consumer Enter
The very best weighted metric is Whole Blocking Time (TBT), a metric that appears on the whole time after the First Contentful Paint (FCP) to assist point out the place the principle thread could also be blocked lengthy sufficient to stop speedy responses to person enter. The principle thread is taken into account “blocked” any time there’s a JavaScript activity operating on the principle thread for greater than 50ms. Minimizing TBT ensures that an internet web page responds to bodily person enter (e.g., key presses, mouse clicks, and so forth).
2. A Net Web page Ought to Load Helpful Content material With No Surprising Visible Shifts
The subsequent most weighted Lighthouse metrics are Largest Contentful Paint (LCP) and Cumulative Structure Shift (CLS). LCP marks the purpose within the web page load timeline when the web page’s major content material has seemingly loaded and is due to this fact helpful.
On the level the place the principle content material has seemingly loaded, you additionally need to preserve visible stability to make sure that customers can use the web page and will not be affected by sudden visible shifts (CLS). A superb LCP rating is something lower than 2.5 seconds (which is quite a bit increased than we’d have thought, given we are sometimes making an attempt to make our web sites as quick as potential).
3. A Net Web page Ought to Load One thing
The First Contentful Paint (FCP) metric marks the primary level within the web page load timeline the place the person can see one thing on the display screen, and the Velocity Index (SI) measures how rapidly content material is visually displayed throughout web page load over time till the web page is “full”.
Your web page is scored based mostly on the velocity indices of actual web sites utilizing efficiency information from the HTTP Archive. A superb FCP rating is lower than 1.8 seconds and SI rating is lower than 3.4 seconds. Each of those thresholds are increased than you would possibly count on when eager about velocity.
Usability Is Favored Over Uncooked Velocity
Google Lighthouse’s efficiency scoring is, definitely, much less about velocity and extra about usability. Your SI and FCP might be tremendous fast, but when your LCP takes too lengthy to color, and if CLS is attributable to massive photographs or exterior content material taking a while to load and shifting issues visually, then your general efficiency rating will probably be decrease than in case your web page was somewhat slower to render the FCP however didn’t trigger any CLS. In the end, if the web page is unresponsive as a consequence of JavaScript blocking the principle thread for greater than 50ms, your efficiency rating will undergo greater than if the web page was somewhat sluggish to color the FCP.
To grasp extra about how the weightings of every metric contribute to the ultimate efficiency rating, you possibly can play about with the sliders on the Lighthouse Scoring Calculator, and right here’s a rudimentary desk demonstrating the impact of skewed particular person metric weightings on the general efficiency rating, proving that web page usability and responsiveness is favored over uncooked velocity.
Description
FCP (ms)
SI (ms)
LCP (ms)
TBT (ms)
CLS
General Rating
Sluggish to point out one thing on display screen
6000
0
0
0
0
90
Sluggish to load content material over time
0
5000
0
0
0
90
Sluggish to load the most important a part of the web page
0
0
6000
0
0
76
Visible shifts occurring throughout web page load
0
0
0
0
0.82
76
Web page is unresponsive to person enter
0
0
0
2000
0
70
The general Google Lighthouse efficiency rating is calculated by changing every uncooked metric worth right into a rating from 0 to 100 based on the place it falls on its Lighthouse scoring distribution, which is a log-normal distribution derived from the efficiency metrics of actual web site efficiency information from the HTTP Archive. There are two major takeaways from this mathematically overloaded info:
Your Lighthouse efficiency rating is plotted towards actual web site efficiency information, not in isolation.
On condition that the scoring makes use of log-normal distribution, the connection between the person metric values and the general rating is non-linear, which means you may make substantial enhancements to low-performance scores fairly simply, but it surely turns into tougher to enhance an already excessive rating.
Learn extra about how metric scores are decided, together with a visualization of the log-normal distribution curve on developer.chrome.com.
Can We “Trick” Google Lighthouse?
I admire Google’s give attention to usability over pure velocity within the internet efficiency dialog. It urges builders to assume much less about aiming for uncooked numbers and extra about the actual experiences we construct. That being mentioned, I’ve puzzled whether or not at the moment in 2024, it’s potential to idiot Google Lighthouse into believing {that a} unhealthy web page when it comes to usability and usefulness is definitely an important one.
I placed on my lab coat and science goggles to analyze. All assessments have been carried out:
Utilizing the Chromium Lighthouse plugin,
In an incognito window within the Arc browser,
Utilizing the “navigation” and “cell” settings (aside from the place described in a different way),
By me, in a lab (i.e., no area information).
That every one being mentioned, I totally acknowledge that my managed check setting contradicts my recommendation on the prime of this publish, however the experiment is an fascinating experience nonetheless. What I hope you’ll take away from that is that Lighthouse scores are just one piece — and a tiny one at that — of a really massive and complicated internet efficiency puzzle. And, with out area information, I’m undecided any of this issues anyway.
The way to Hack FCP and LCP Scores
TL;DR: Present the smallest quantity of LCP-qualifying content material on load to spice up the FCP and LCP scores till the Lighthouse check has seemingly completed.
FCP marks the primary level within the web page load timeline the place the person can see something in any respect on the display screen, whereas LCP marks the purpose within the web page load timeline when the principle web page content material (i.e., the most important textual content or picture component) has seemingly loaded. A quick LCP helps reassure the person that the web page is helpful. “Seemingly” and “helpful” are the vital phrases to keep in mind right here.
What Counts as an LCP Factor
The sorts of parts on an online web page thought of by Lighthouse for LCP are:
<img> parts,
<picture> parts inside an <svg> component,
<video> parts,
A component with a background picture loaded utilizing the url() operate, (and never a CSS gradient), and
Block-level parts containing textual content nodes or different inline-level textual content parts.
The next parts are excluded from LCP consideration because of the chance they don’t include helpful content material:
Parts with zero opacity (invisible to the person),
Parts that cowl the complete viewport (prone to be background parts), and
Placeholder photographs or different photographs with low entropy (i.e., low informational content material, equivalent to a solid-colored picture).
Nonetheless, the notion of a picture or textual content component being helpful is totally subjective on this case and usually out of the realm of what machine code can reliably decide. For instance, I constructed a web page containing nothing however a <h1> component the place, after 10 seconds, JavaScript inserts extra descriptive textual content into the DOM and hides the <h1> component.
Lighthouse considers the heading component to be the LCP component on this experiment. At this level, the web page load timeline has completed, however the web page’s major content material has not loaded, regardless that Lighthouse thinks it’s seemingly to have loaded inside these 10 seconds. Lighthouse nonetheless awards us with an ideal rating of 100 even when the heading is changed by a single punctuation mark, equivalent to a full cease, which is even much less helpful.
This check means that if you want to load web page content material through client-side JavaScript, we‘ll need to keep away from displaying a skeleton loader display screen since that requires loading extra parts on the web page. And since we all know the method will take a while — and that we will offload the community request from the principle thread to an online employee so it received’t have an effect on the TBT — we will use some arbitrary “splash display screen” that comprises a minimal viable LCP component (for higher FCP scoring). This fashion, we’re giving Lighthouse the impression that the web page is beneficial to customers faster than it truly is.
All we have to do is embody a sound LCP component that comprises one thing that counts because the FCP. Whereas I might by no means advocate loading your major web page content material through client-side JavaScript in 2024 (serve static HTML from a CDN as a substitute or construct as a lot of the web page as you possibly can on a server), I might undoubtedly not advocate this “hack” for person expertise, no matter what the Lighthouse efficiency rating tells you. This strategy additionally received’t earn you any favors with engines like google indexing your web site, because the robots are unable to find the principle content material whereas it’s absent from the DOM.
I additionally tried this experiment with quite a lot of random photographs representing the LCP to make the web page even much less helpful. However provided that I used small file sizes — made smaller and transformed into “next-gen” picture codecs utilizing a third-party picture API to assist with web page load velocity — it appeared that Lighthouse interpreted the weather as “placeholder photographs” or photographs with “low entropy”. Consequently, these photographs have been disqualified as LCP parts, which is an effective factor and makes the LCP barely much less hackable.
View the demo web page and use Chromium DevTools in an incognito window to see the outcomes your self.
This hack, nonetheless, in all probability received’t maintain up in lots of different use instances. Discord, for instance, makes use of the “splash display screen” strategy whenever you hard-refresh the app within the browser, and it receives a tragic 29 efficiency rating.
In comparison with my DOM-injected demo, the LCP component was calculated as some content material behind the splash display screen slightly than parts contained inside the splash display screen content material itself, given there have been a number of massive photographs within the focussed textual content channel I examined on. One may argue that Lighthouse scores are much less vital for apps which can be behind authentication anyway: they don’t should be listed by engines like google.
There are seemingly many different conditions the place apps serve user-generated content material and also you may be unable to manage the LCP component totally, significantly concerning photographs.
For instance, when you can management the sizes of all the photographs in your internet pages, you would possibly have the ability to make the most of an fascinating hack or “optimization” (in very massive quotes) to arbitrarily recreation the system, as was the case of RentPath. In 2021, builders at RentPath managed to enhance their Lighthouse efficiency rating by 17 factors when growing the scale of picture thumbnails on an online web page. They satisfied Lighthouse to calculate the LCP component as one of many bigger thumbnails as a substitute of a Google Map tile on the web page, which takes significantly longer to load through JavaScript.
The underside line is you can acquire increased Lighthouse efficiency scores in case you are conscious of your LCP component and in command of it, whether or not that’s via a hack like RentPath’s or mine or a real-deal enchancment. That being mentioned, while I’ve described the splash display screen strategy as a hack on this publish, that doesn’t imply this kind of expertise couldn’t provide a purposeful and joyful expertise. Efficiency and person expertise are about understanding what’s taking place throughout web page load, and it’s additionally about intent.
The way to Hack CLS Scores
TL;DR: Defer loading content material that causes format shifts till the Lighthouse check has seemingly completed to make the check assume it has sufficient information. CSS transforms don’t negatively impression CLS, besides if used along side new parts added to the DOM.
CLS is measured on a decimal scale; rating is lower than 0.1, and a poor rating is larger than 0.25. Lighthouse calculates CLS from the most important burst of sudden format shifts that happen throughout a person’s time on the web page based mostly on a mixture of the viewport measurement and the motion of unstable parts within the viewport between two rendered frames. Smaller one-off situations of format shift could also be inconsequential, however a bunch of format shifts taking place one after the opposite will negatively impression your rating.
If you recognize your web page comprises annoying format shifts on load, you possibly can defer them till after the web page load occasion has been accomplished, thus fooling Lighthouse into pondering there isn’t any CLS. This demo web page I created, for instance, earns a CLS rating of 0.143 regardless that JavaScript instantly begins including new textual content parts to the web page, shifting the unique content material up. By pausing the JavaScript that provides new nodes to the DOM by an arbitrary 5 seconds with a setTimeout(), Lighthouse doesn’t seize the CLS that takes place.
This different demo web page earns a efficiency rating of 100, regardless that it’s arguably much less helpful and useable than the final web page provided that the added parts pop in seemingly at random with none person interplay.
While it’s potential to defer format shift occasions for a web page load check, this hack undoubtedly received’t work for area information and person expertise over time (which is a extra vital focus, as we mentioned earlier). If we carry out a “time span” check in Lighthouse on the web page with deferred format shifts, Lighthouse will appropriately report a non-green CLS rating of round 0.186.
In the event you do need to deliberately create a chaotic expertise just like the demo, you need to use CSS animations and transforms to extra purposefully pop the content material into view on the web page. In Google’s information to CLS, they state that “content material that strikes progressively and naturally from one place to a different can usually assist the person higher perceive what’s occurring and information them between state modifications” — once more, highlighting the significance of person expertise in context.
On this subsequent demo web page, I’m utilizing CSS rework to scale() the textual content parts from 0 to 1 and transfer them across the web page. The transforms fail to set off CLS as a result of the textual content nodes are already within the DOM when the web page masses. That mentioned, I did observe in my testing that if the textual content nodes are added to the DOM programmatically after the web page masses through JavaScript and then animated, Lighthouse will certainly detect CLS and rating issues accordingly.
You Can’t Hack a Velocity Index Rating
The Velocity Index rating is predicated on the visible progress of the web page because it masses. The faster your content material masses nearer the start of the web page load timeline, the higher.
It’s potential to do some hack to trick the Velocity Index into pondering a web page load timeline is slower than it’s. Conversely, there’s no actual technique to “faux” loading content material quicker than it does. The one technique to make your Velocity Index rating higher is to optimize your internet web page for loading as a lot of the web page as potential, as quickly as potential. While not totally life like within the internet panorama of 2024 (primarily as a result of it will put designers out of a job), you may go all-in to decrease your Velocity Index as a lot as potential by:
Delivering static HTML internet pages solely (no server-side rendering) straight from a CDN,
Avoiding photographs on the web page,
Minimizing or eliminating CSS, and
Stopping JavaScript or any exterior dependencies from loading.
You Additionally Can’t (Actually) Hack A TBT Rating
TBT measures the overall time after the FCP the place the principle thread was blocked by JavaScript duties for lengthy sufficient to stop responses to person enter. A superb TBT rating is something decrease than 200ms.
JavaScript-heavy internet purposes (equivalent to single-page purposes) that carry out complicated state calculations and DOM manipulation on the consumer on web page load (slightly than on the server earlier than sending rendered HTML) are susceptible to struggling poor TBT scores. On this case, you may in all probability hack your TBT rating by deferring all JavaScript till after the Lighthouse check has completed. That mentioned, you’d want to supply some form of placeholder content material or loading display screen to fulfill the FCP and LCP and to tell customers that one thing will occur sooner or later. Plus, you’d should go to additional lengths to hack across the front-end framework you’re utilizing. (You don’t need to load a placeholder web page that, sooner or later within the web page load timeline, masses a separate React app after an arbitrary period of time!)
What’s fascinating is that whereas we’re nonetheless doing all types of fancy issues with JavaScript within the consumer, advances within the fashionable internet ecosystem are serving to us all cut back the chance of a less-than-stellar TBT rating. Many front-end frameworks, in partnership with fashionable internet hosting suppliers, are able to rendering pages and processing complicated logic on demand with none client-side JavaScript. Whereas eliminating JavaScript on the consumer shouldn’t be the purpose, we actually have quite a lot of choices to make use of quite a bit much less of it, thus minimizing the danger of doing an excessive amount of computation on the principle thread on web page load.
Backside Line: Lighthouse Is Nonetheless Simply A Tough Information
Google Lighthouse can’t detect all the things that’s incorrect with a selected web site. While Lighthouse efficiency scores prioritize web page usability when it comes to responding to person enter, it nonetheless can’t detect each horrible usability or accessibility problem in 2024.
In 2019, Manuel Matuzović printed an experiment the place he deliberately created a horrible web page that Lighthouse thought was fairly nice. I hypothesized that 5 years later, Lighthouse would possibly do higher; but it surely doesn’t.
On this remaining demo web page I put collectively, enter occasions are disabled by CSS and JavaScript, making the web page technically unresponsive to person enter. After 5 seconds, JavaScript flips a change and means that you can click on the button. The web page nonetheless scores 100 for each efficiency and accessibility.
You actually can’t depend on Lighthouse as an alternative choice to usability testing and customary sense.
Some Extra Foolish Hacks
As with all the things in life, there’s at all times a technique to recreation the system. Listed below are some extra tried and examined assured hacks to ensure your Lighthouse efficiency rating artificially knocks everybody else’s out of the park:
Solely run Lighthouse assessments utilizing the quickest and highest-spec {hardware}.
Make certain your web connection is the quickest it may be; relocate if you want to.
By no means use area information, solely lab information, collected utilizing the aforementioned quickest and highest-spec {hardware} and super-speed web connection.
Rerun the assessments within the lab utilizing completely different circumstances and all of the particular code hacks I described on this publish till you get the outcome(s) you need to impress your pals, colleagues, and random folks on the web.
Be aware: One of the best ways to find out about internet efficiency and easy methods to optimize your web sites is to do the exact opposite of all the things we’ve lined on this article the entire time. And at last, to noticeably degree up your efficiency abilities, use an utility monitoring software like Sentry. Consider Lighthouse because the canary and Sentry because the real-deal production-data-capturing, lean, imply, internet vitals machine.
And at last-finally, right here’s the hyperlink to the complete demo web site for academic functions.
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