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Testing your website's speed, and what the numbers mean
PageSpeed Insights, Google's free speed test, shows two different things on one page: how real visitors' phones experienced your site over the past four weeks, and a single simulated test. They can disagree, and only one of them is about your visitors.
By Oscar CobbeCurrent as at 2 minute read3 sources
The two halves of the report
The top half is field data: what Chrome users visiting your site experienced, taken from the Chrome User Experience Report over the previous 28 days.
The bottom half is lab data: one test run by Lighthouse in a simulated setting, which for mobile means a mid-range phone on a mobile network. The performance score out of 100 comes from this run, with 90 and above counted as good and under 50 as poor.
Google's own explanation of why they differ is that the lab test is one device on one set of network conditions, while the field data covers every real device and connection your visitors used.
The three numbers that decide a pass
Google's Core Web Vitals are three measurements, each with a threshold for good. A page passes when all three are good at the 75th percentile of real page loads, measured separately for mobile and desktop.
| Measurement | What it measures | Good is |
|---|---|---|
| Largest Contentful Paint (LCP) | How long until the biggest thing on the screen, usually the main image or heading, has appeared | 2.5 seconds or less |
| Interaction to Next Paint (INP) | How quickly the page responds when someone taps or clicks | 200 milliseconds or less |
| Cumulative Layout Shift (CLS) | How much the page jumps about while it loads | 0.1 or less |
INP replaced FID
Interaction to Next Paint replaced First Input Delay as a Core Web Vital in 2024. An older report or a guide that still talks about FID is measuring something Google no longer uses.
What to do with the result
A score is a starting point. Four things to do with it:
- 1Test the pages people land on, not only the homepage: a service page or a price page is often the first one a visitor from a search sees.
- 2Test on mobile first: it is the harder of the two tests, and where a slow server shows most. What hosting includes covers the server side.
- 3Fix the largest item first. A slow LCP can come down to one oversized image, and a high CLS to an image or a banner that loads with no space set aside for it.
- 4Look at the field data again after a change, not the next morning: it is a rolling four-week window and moves slowly.
Speed and the rest of the build
A page that jumps while someone is about to tap a button sends the tap to the wrong place, which is a usability failure before it is a speed one. Every site we build is timed on a phone before it goes live, and the client is given the figure. What a website costs sets that against what else a build should include.
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See the Websites pageWho wrote this
Oscar Cobbe · Founder, FourWinds Digital
Writes and maintains the legal explainers on this site, and does the compliance work behind them. Every date and article number here is checked against the legislation itself before it is published, and each article shows the date it was last checked.
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Written on 29 October 2026; it describes the law as FourWinds read it on that date. This is general information about how the rules work, not legal advice on your situation. Where you need a solicitor, we say so. How we check the law.