Core Web Vitals are three measurements Google uses to judge real visitors' experience of a page: how fast the main content appears (LCP), how quickly the page responds to input (INP) and how much the layout jumps around (CLS). A page passes when at least 75% of visits meet the "good" threshold for all three.
What are the thresholds?
| Metric | Measures | Good | Poor |
|---|---|---|---|
| Largest Contentful Paint (LCP) | When the largest image or text block appears | 2.5 seconds or less | Over 4 seconds |
| Interaction to Next Paint (INP) | Delay between a tap or click and the screen updating | 200 ms or less | Over 500 ms |
| Cumulative Layout Shift (CLS) | How much visible content moves unexpectedly | 0.1 or less | Over 0.25 |
INP replaced First Input Delay as a Core Web Vital in March 2024. The definitions are on web.dev.
Lab data or field data?
Field data comes from real Chrome users through the Chrome UX Report, and it is what Google uses. Lab data, such as a Lighthouse score, is a single simulated load: useful for finding causes, but it can differ from what visitors experience. Fix with lab data, judge success with field data.
How do you improve LCP?
- Make the server respond fast: aim for a time to first byte under about 0.8 seconds, using caching and a CDN.
- Do not lazy-load the main image above the fold; load it early, with
fetchpriority="high". - Serve images in modern formats such as WebP or AVIF, at the size they are displayed.
- Remove render-blocking CSS and JavaScript from the
<head>, or load scripts withdefer. - Turn on Brotli or Gzip compression and long cache lifetimes for static files.
How do you improve INP?
- Break up long JavaScript tasks so the browser can respond between them.
- Load less third-party script: chat widgets, tag managers and ad scripts are frequent causes.
- Do less work in click and input handlers; update the screen first, then do the rest.
How do you improve CLS?
- Give images, videos and iframes
widthandheightattributes so space is reserved. - Reserve space for ads, banners and embeds before they load.
- Avoid inserting content above existing content, except in response to a user action.
- Use
font-display: optionalor preload web fonts to avoid text jumping when fonts load.
Do Core Web Vitals affect rankings?
They are part of Google's page experience signals. Relevance and content quality matter more, so a fast page with weak content will not outrank a slower page that answers the question better. But between similar pages, speed can make the difference, and faster pages convert more visitors either way.
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