You type your address into Google PageSpeed Insights, wait twenty seconds, and get a number out of 100. It looks like a grade for your website. It is something narrower than that, and knowing what it measures tells you what to fix first.
One simulated visit, on a phone
The score comes from Lighthouse, a testing tool Google runs on its own servers. It loads your page once, pretending to be a mid-range phone on a slow mobile connection: the processor is slowed down and the network is throttled. That is deliberate. It is close to how a large share of your visitors meet your site, and far from your office laptop on fast Wi-Fi.
Because it is one simulated visit, it is called lab data. It is repeatable enough to compare before and after a change, which is what it is best at.
Five metrics, weighted
The performance score is a weighted average of five measurements, each turned into a 0 to 100 score first. In the current version of Lighthouse the weights are:
| Metric | What it measures | Weight |
|---|---|---|
| Total Blocking Time | How long the page is too busy to respond to a tap | 30% |
| Largest Contentful Paint | When the main content, usually the hero image or headline, appears | 25% |
| Cumulative Layout Shift | How much the page jumps around while it loads | 25% |
| First Contentful Paint | When anything at all appears | 10% |
| Speed Index | How quickly the visible page fills in | 10% |
Read that table again: more than half the score is about the page being responsive and stable, not only fast. A page that appears quickly but freezes for two seconds while scripts load, or shifts the button just as someone taps it, scores badly, and it should.
What the colours mean
Google colours the score: 90 to 100 is good, 50 to 89 needs improvement, and 0 to 49 is poor. The same bands are used for each metric. A small business site with a large slider, a chat widget and several tracking scripts usually lands in the red on mobile, even when it feels fine on a desktop.
Core Web Vitals are a different thing
Above the lab score, PageSpeed Insights sometimes shows a second section with real-user data. These are the Core Web Vitals, collected from Chrome users who visited your site over the last 28 days. They only appear when your site has enough traffic for Google to report.
Google treats a page as good when, for at least 75 percent of visits:
- Largest Contentful Paint is 2.5 seconds or less;
- Interaction to Next Paint, how quickly the page responds to taps and clicks, is 200 milliseconds or less;
- Cumulative Layout Shift is 0.1 or less.
These field numbers are what Google uses as a page experience signal in search. The lab score is the tool you use to find and fix the causes.
Why the score changes between runs
Run the test three times and you may get three different numbers. Server response time, third-party scripts that load differently each time, and the test machine itself all vary. A change of a few points means nothing; a change of twenty means something changed. When we measure before and after a project, we run the test several times and compare like with like.
Which number to act on
- Start with the mobile result, not desktop.
- Look at the metric with the worst colour, not the overall score.
- Read the diagnostics below the score: they name the image, script or font responsible.
- If you have field data, that is the one that counts for your visitors and for search.
Our free site check runs this same Google test on the mobile version of your site and shows the score with the four numbers that matter most.