Core Web Vitals: the three numbers that decide whether Google shows your site
LCP, INP and CLS sound like jargon until you realise they are just measurements of whether your site feels broken. Here is what each one means in plain English.
Google measures three things about how your pages feel to real visitors, and uses them as a ranking signal. They have intimidating names and very simple meanings.
LCP — how long until the page looks ready
Largest Contentful Paint measures when the biggest thing on screen — usually your hero image or headline — has finished rendering. Google wants this under 2.5 seconds.
The usual culprits are enormous unoptimised images, fonts that block rendering, and hero sections that wait for JavaScript before they draw anything.
INP — how long until the page responds to you
Interaction to Next Paint measures the delay between a visitor tapping something and the page visibly reacting. Target is under 200ms.
This is the one that punishes heavy JavaScript. If your main thread is busy running analytics, a chat widget, and three A/B testing scripts, the tap has to queue behind all of it.
CLS — how much the page jumps around
Cumulative Layout Shift measures content moving after it has appeared. You have felt this: you go to tap a link, an ad loads above it, and you tap something else. Target is under 0.1.
It is almost always caused by images without dimensions, web fonts swapping in at a different size, or banners injected at the top of the page after load.
What to actually do about it
- 01Measure real visitors, not your laptop. Use the Chrome UX Report data in PageSpeed Insights — your machine on office wifi is not your customer on a train.
- 02Fix images first. It is the highest-leverage change on almost every site.
- 03Audit third-party scripts ruthlessly. Every tag someone added “just to test” is still there.
- 04Set explicit width and height on everything, including ads and embeds.
Worth saying plainly: passing Core Web Vitals will not rescue a site nobody wants to read. It removes a penalty. It does not create demand.
We hold ourselves to a performance budget on every build, and we re-check it after launch — a site that scores 100 on launch day and 60 six months later has not really scored 100.
