Core Web Vitals, explained without the jargon

Google ranks pages partly on three measurements called Core Web Vitals. The names are unfriendly — LCP, INP, CLS — but the ideas behind them are things you already notice as a user every day.

LCP — how long until the main thing shows up

Largest Contentful Paint is the time until the biggest element on screen — usually your hero image or headline — actually appears. It's the moment the page stops feeling blank. Good is under 2.5 seconds.

What usually ruins it: an enormous unoptimised hero image, slow server response, or CSS and scripts that block the browser from painting. This is typically the easiest vital to improve, because the top offender is so often one oversized image.

INP — how quickly the page answers you

Interaction to Next Paint measures the delay between you tapping something and the page visibly responding. Ever hit a menu button and had nothing happen for a second? That's bad INP. Good is under 200 milliseconds.

The usual cause is too much JavaScript doing too much work — often from plugins loading site-wide for features used on one page.

CLS — does the page jump around

Cumulative Layout Shift scores how much the layout moves while loading. It's the reason you sometimes tap a button and hit the ad that just shoved itself into that spot. Good is under 0.1.

Causes are mundane: images without width and height attributes, late-loading banners and embeds, fonts swapping size. Mundane causes have mundane fixes — which is good news.

Two things worth knowing before you chase scores

• Google uses real-visitor data (mostly mobile), not the lab number you get from a single test run on office wifi. Optimise for the phone on mediocre reception, because that's who Google is averaging.
• A perfect 100 isn't the goal. Moving from "poor" to "good" is where the ranking and conversion benefit lives; the last ten points cost the most and pay the least. Be suspicious of anyone guaranteeing 95+ before they've profiled your site.

Check your own numbers free at pagespeed.web.dev — and if they're red, the fixes above are exactly what a proper optimisation pass works through, in that order.