Why Core Web Vitals exist
Traditional performance metrics like total load time don’t capture what a visitor actually experiences — a page can ‘finish loading’ technically while still feeling slow or unstable to use. Core Web Vitals were designed to measure the parts of the experience that visitors actually notice.
LCP — Largest Contentful Paint
LCP measures how long it takes for the largest visible element — usually a hero image, a block of text, or a video poster — to render on screen. A good LCP is under 2.5 seconds. Slow LCP is usually caused by slow server response, render-blocking resources, or an unoptimized hero image.
INP — Interaction to Next Paint
INP measures how long it takes for the page to visually respond after a visitor interacts with it — a click, a tap, a key press. A good INP is under 200 milliseconds. Slow INP is usually caused by heavy JavaScript execution blocking the browser’s main thread.
CLS — Cumulative Layout Shift
CLS measures how much visible content unexpectedly moves while a page loads — images without reserved space, ads that inject late, or fonts that swap and reflow text. A good CLS score is under 0.1. It’s usually fixed by explicitly sizing images and reserving space for dynamic content.
How to check your Core Web Vitals
Google Search Console reports field data (real visitor data) for your site under its Core Web Vitals report. PageSpeed Insights shows both field data (where available) and lab data from a single test run — field data is the more accurate reflection of real user experience over time.