Web Check · feature

Website speed test (lab Lighthouse scores and real-user Core Web Vitals)

A website speed test in Web Check looks at speed from two sides at once: a lab measurement of the page and the experience of real visitors. You enter a page address and get Lighthouse scores, Core Web Vitals from Chrome users where such data exists, and a review of how the server delivers files: compression, caching, HTTP/3 and CDN.

Website speed test report: four Lighthouse scores and a delivery review

What a website speed test measures

Three checks work together, and the report keeps their results apart:

Lab scores. quality runs Lighthouse and returns four categories, Performance, Accessibility, Best Practices and SEO, each from 0 to 100.
Real visitors. crux brings Core Web Vitals from the Chrome UX Report through the PageSpeed Insights API.
Delivery. delivery examines response compression, caching of static files, the HTTP/3 sign and the CDN in use.

Extra checks round out the picture: redirects (the chain of redirects), carbon (an estimate of the carbon footprint of the page) and pwa.

How the numbers are collected, step by step

A website speed test runs as a chain of five actions.

You enter a page address, or call the API with the url parameter.
quality runs Lighthouse on an emulated device and returns the four category scores.
crux asks the PageSpeed Insights API for Core Web Vitals of real Chrome visitors.
delivery reads the compression of the response (gzip, br or zstd) and looks for an Alt-Svc sign of HTTP/3 and for a CDN. Caching is measured on real static resources of the page: the first css file, the first js file and the first image.
The summary turns weak numbers into remarks: a category below 90 is a remark, below 70 the remark gets a higher importance.

Lab data against real users

A website speed test draws on two sources that answer different questions, and mixing them would blur both. Lighthouse is a laboratory: it runs the page on an emulated device. The Chrome UX Report describes what real visitors experience.

The report shows both, one next to the other, without mixing them. When a site gets little traffic from Chrome, the field part says "not enough data". That is a fact about the traffic and not a failure of the check, and the lab scores stay available.

What you get from the report

Four Lighthouse scores, each from 0 to 100, with remarks for every category under 90.
Core Web Vitals of real users, when the Chrome UX Report has enough data for the site.
A delivery review: compression, caching of static files, HTTP/3 and CDN.
Compression is read as gzip, br or zstd, and br and zstd are the preferred ones.
Static files without cache headers give a remark of high importance; when only some of the resources lack them, the remark is of medium importance.

Caching is measured on the actual static resources of the page, not only on the headers of the HTML document.

Where it helps and how to run it

Before a release. A website speed test shows which Lighthouse category is below 90 and what the remark says.
After changing hosting or CDN. delivery shows whether compression and caching survived the move.
On a young site. When the Chrome UX Report has too little data, you work with the lab scores.

Each check is a separate call:

GET https://webcheck.asibots.pro/api/quality?url=PAGE_URL
GET https://webcheck.asibots.pro/api/crux?url=PAGE_URL
GET https://webcheck.asibots.pro/api/delivery?url=PAGE_URL

Limits you should know about

The Lighthouse check is heavy, so the service allows no more than 12 of them per minute from one visitor. Above that the answer is 429 with a clear text.
Lighthouse results come from an emulated device in a lab, so they do not replace the numbers of your own visitors.
The Chrome UX Report part depends on traffic from Chrome. For a site without enough of it, "not enough data" is the normal answer.
A full audit takes around 40 seconds, and a response through Cloudflare is limited to 100 seconds.

The website speed test is one part of the full audit, so its results also appear in the combined report next to security and SEO sections.

Frequently asked questions

Why does the report say "not enough data" for Core Web Vitals?

Real-user numbers come from the Chrome UX Report through the PageSpeed Insights API. If a site has little traffic from Chrome, there is nothing to report. It is not an error of the check.

What is the difference between Lighthouse and Chrome UX Report data?

Lighthouse is a lab run on an emulated device. The Chrome UX Report reflects real visitors. Web Check shows both on one page and does not mix them.

How does a website speed test check caching?

On real static resources of the page: the first css, js and image. The check does not rely only on the headers of the HTML document alone. Static files with no cache headers are a high-importance remark.

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