What Google actually measures when it measures your site speed
If you have ever run your site through Google’s PageSpeed tool, you got a number out of 100 and a wall of red. Somewhere underneath were three acronyms — LCP, INP, CLS — and a lot of language that assumed you already knew what they meant.
Here is the part nobody leads with: that score out of 100 is not what Google ranks you on. The three acronyms are. They measure different things, they can disagree completely, and only one of them is used for search.
This post explains all of it in plain language, using real measurements from real sites.
The three things Google measures
Google calls them Core Web Vitals. Each one measures a different kind of frustration.
Loading — “Largest Contentful Paint”
How long until the main thing on the page shows up. Not the first pixel, not when the loading spinner stops: the moment the biggest piece of content — usually your hero image or headline — is actually on screen.
| Good | 2.5 seconds or less |
| Needs work | 2.5 – 4 seconds |
| Poor | over 4 seconds |
Responsiveness — “Interaction to Next Paint”
How long the page takes to react when someone taps something. They tap a button; how long before anything visibly happens? This is the metric behind that feeling of a page being “laggy” — you tap, nothing moves, you tap again, and then both taps register.
| Good | 200 milliseconds or less |
| Needs work | 200 – 500 ms |
| Poor | over 500 ms |
If you have read about this before and remember something called FID, that was the previous version. INP replaced it as a Core Web Vital in 2024, and it is stricter — FID only measured the delay before the first interaction, while INP watches the whole visit.
Stability — “Cumulative Layout Shift”
How much the page moves around while it loads. You go to tap a link, an image finishes loading above it, everything jumps down, and you tap an advert instead. That is layout shift, and it is measured as a score rather than a time.
| Good | 0.1 or less |
| Needs work | 0.1 – 0.25 |
| Poor | over 0.25 |
The bit that trips everyone up: 75th percentile
Google does not use your average. It uses the 75th percentile of real page loads — meaning three out of four visits must hit the target.
This matters more than it sounds. Averages hide your worst experiences: if most visitors are on fast laptops on office wifi and a quarter are on mid-range phones on mobile data, the average looks fine and a quarter of your customers are having a bad time. Google deliberately measures so that you cannot average your way out of it.
It is also assessed separately for mobile and desktop. Passing on desktop and failing on mobile is a fail where it counts, because most people are on phones.
Lab data and field data are not the same thing
This is the single biggest source of confusion, and it explains almost every “but my score says…” argument.
Lab data is a simulation. Google loads your page on a pretend mid-range phone over a pretend slow connection, in a data centre. It is repeatable, available for any URL instantly, and it is what produces that score out of 100.
Field data is real. It comes from actual Chrome users visiting your site over the past 28 days, on their real devices and real connections. This is what Google uses for search ranking.
Two consequences people find surprising:
A new or quiet site has no field data at all. Google needs enough traffic before it will report on you. Below that threshold you only get the simulation — which tells you how the page is built, not what anyone experiences.
The two can disagree wildly. Which brings us to the example.
A real site that scores 19 and loads fine
Here is a car dealership’s About page, measured on mobile. Its Lighthouse score is 19 out of 100 — the sort of number that makes people panic.
Now the field data, from real visitors:
| Metric | Real visitors | Verdict |
|---|---|---|
| Loading (LCP) | 1.7 s | Good |
| Responsiveness (INP) | 121 ms | Good |
| Stability (CLS) | 0.40 | Poor |
Read that again. A site scoring 19 out of 100 is loading its main content in 1.7 seconds and responding to taps in 121 milliseconds. For real people, on real phones, it is not slow.
It has exactly one problem: it jumps around. A CLS of 0.40 is four times the threshold. Visitors go to tap something and the page moves under their finger.
The panic-inducing score of 19 would have you rebuilding the site. The field data says: reserve space for your images and you are largely done.
For contrast, here is Wikipedia’s homepage on the same day:
| Metric | Real visitors | Verdict |
|---|---|---|
| Loading (LCP) | 1.2 s | Good |
| Responsiveness (INP) | 96 ms | Good |
| Stability (CLS) | 0.00 | Good |
Lighthouse score: 100. When a site is genuinely well built, the score and the field data agree. It is when they disagree that you learn something.
So what should you actually do?
Look at your field data first, not your score. Google Search Console has a Core Web Vitals report showing exactly what real visitors experience, free, for any site you own. If you only ever check one thing, check that.
If you have no field data, you do not have a traffic problem to solve yet. Use the lab score as a build-quality check and get on with the rest of your business.
Fix the metric that is actually failing. Our dealership does not need a faster server — its loading is fine. It needs images with dimensions set. Three metrics, three completely different sets of fixes, and doing the wrong one costs money and changes nothing.
Treat the score out of 100 as a smoke alarm, not a diagnosis. It is useful for noticing that something is worth investigating. It is not what Google ranks you on, and as the dealership shows, it can be dramatically more alarming than reality.
The short version
- Three metrics: loading, responsiveness, stability
- Thresholds: 2.5 s, 200 ms, 0.1
- Measured at the 75th percentile — three in four visits must pass
- Real-visitor data is what counts for ranking, not the simulated score
- A bad score does not always mean a slow site, and a fast site can still fail
If you want to know which of the three is your problem, the free check on this site will tell you in plain language — and it will tell you whether Google has real-visitor data for you yet, which is the first thing worth knowing.