Core Web Vitals in plain English
What LCP, CLS and INP actually measure, why your score moves when you didn't change anything, and which of the three is worth your money first.
Core Web Vitals are Google's three measurements of how a page feels to a real visitor. They affect search rankings a little, and conversions a lot. Here's what each one means without the jargon.
LCP — how long until the main thing shows up
Largest Contentful Paint measures when the biggest visible element — usually the hero image or main heading — finishes loading. Good is under 2.5 seconds.
Typical fixes: compress the hero image, stop lazy-loading it, improve server response time, and remove render-blocking CSS and fonts.
CLS — does the page jump around
Cumulative Layout Shift measures how much content moves while loading. You've felt it: you go to tap a button, an ad loads above it, and you tap the wrong thing. Good is under 0.1.
Typical fixes: set width and height on images and embeds, reserve space for banners and cookie notices, and avoid injecting content above what's already visible.
INP — does the page respond when I tap
Interaction to Next Paint replaced FID in 2024. It measures how quickly the page visibly reacts after a click, tap or key press — across the whole visit, not just the first interaction. Good is under 200 milliseconds.
Typical fixes: reduce heavy JavaScript, defer third-party scripts (chat widgets, trackers), and break up long tasks.
Why your score changes when you didn't change anything
- Lab vs field data. PageSpeed's top section is real-user data from the last 28 days; the lower section is a single simulated test. They will disagree.
- Your visitors changed. A campaign that brings mobile users on slow connections will lower your field scores.
- Third parties changed. A marketing script, chat widget or ad network updated their code.
- Plugins updated. A plugin started loading assets on every page.
Which one to fix first
For most WordPress sites I work on, LCP is the best return on investment. It's usually caused by a few concrete things — images, hosting, caching — and fixing it improves how fast the whole site feels.
CLS is often a quick win too. INP tends to be the hardest, because the fix usually means removing scripts someone in marketing is attached to.
Fix what visitors feel first. The score follows.