How to read the three Core Web Vitals numbers
Google judges your site on three measurements: LCP, CLS and INP. Here is what each one means and what to do when it comes back poor.
You put your site into PageSpeed Insights and got a colourful report back. Most people only look at the score in the circle. Google doesn't use that score for ranking. It looks at the three measurements underneath.
Those three are the Core Web Vitals. Each one measures a different annoyance your visitors run into.
LCP, when the page looks loaded
Largest Contentful Paint measures when the biggest element on screen appears. That's usually a hero image or a large heading.
Good: under 2.5 seconds. Poor: over 4 seconds.
If it's poor
Shrink the largest image and convert it to WebP. This is the fix that works most often
Mark that image as a priority so the browser fetches it first
Check the server's first response time. If it's above 600 ms, look at the server before you touch the code
CLS, the page jumping around
Cumulative Layout Shift measures content moving while you read. You're about to tap a link, an advert loads above it, everything slides down and you hit the wrong thing. Everyone has had it happen.
Good: under 0.1. Poor: over 0.25.
If it's poor
Give every image a width and height so the browser reserves the space up front
Reserve a fixed height for anything that loads late, such as adverts, embedded video or notice bars
Load fonts with font-display: swap and pick a fallback font with similar proportions
INP, how quickly a tap gets a response
Interaction to Next Paint measures how long the screen takes to react when you press a button. It replaced the older FID metric in 2024 because it reflects real use better.
Good: under 200 milliseconds. Poor: over 500 milliseconds.
If it's poor
JavaScript is almost always what breaks INP. If scripts running at page load keep the main thread busy, the browser can't respond when you tap. On most sites, removing third-party tools you don't actually use fixes this number in one go.
Lab data and field data
PageSpeed Insights has two sections. The top one, "real user experience", is data collected from people who visited your site in the last 28 days, and it's what Google uses for ranking.
The lower "diagnose performance" section is a single test run there and then. It shows you what to fix but doesn't affect ranking. On a new site the top section is empty because not enough visitor data has built up yet.
How often should you check?
Once a month is plenty. Field data covers a 28-day window, so a fix you make today won't show up tomorrow. After a change, give it a few weeks before you look for the effect.
If those three numbers are good, you're done on speed. Chasing a score of 100 after that point gets your visitors nothing.