What are Core Web Vitals? LCP, INP and CLS explained
3 min read below · WebYes knowledge base
Core Web Vitals measure loading speed, responsiveness and visual stability. Learn what LCP, INP and CLS mean and which values score well.
Core Web Vitals are three metrics Google uses to measure the user experience of a web page: LCP (loading speed), INP (responsiveness to interaction) and CLS (visual stability). They count towards search rankings and are the basis of any serious speed measurement.
The three metrics and their thresholds
Largest Contentful Paint (LCP) measures how long it takes for the largest visible element (usually an image or heading) to appear. Interaction to Next Paint (INP) measures how quickly the page responds to clicks, taps and key presses. Cumulative Layout Shift (CLS) measures how much the layout shifts while loading, for instance when a banner pushes the text down.
| Metric | Good | Needs improvement | Poor |
|---|---|---|---|
| LCP (loading) | ≤ 2.5 s | 2.5 - 4 s | > 4 s |
| INP (interaction) | ≤ 200 ms | 200 - 500 ms | > 500 ms |
| CLS (stability) | ≤ 0.1 | 0.1 - 0.25 | > 0.25 |
Google assesses the 75th percentile of page visits: slower devices and connections need to score well too.
The thresholds come from Google's official documentation on web.dev. They apply per page, not per site: your homepage can score well while a product page fails.
Why Core Web Vitals matter
Slow pages cost visitors. Every extra second of load time increases the share of people who leave before the page renders. Google also uses the Core Web Vitals as a ranking signal: with comparable content, the faster page wins. For web shops it counts double, because a stuttering checkout directly affects revenue.
The metrics are deliberately chosen around what a visitor actually notices: how quickly do I see something, how quickly does it respond, and does the layout stay put. A server that responds fast but a page that then spends seconds running scripts rightly scores poorly.
Lab data versus field data
There are two kinds of measurements. Field data comes from real visitors (through the Chrome User Experience Report) and determines how Google judges your page. Lab data comes from a simulated test, such as a scan or Lighthouse run, and is instantly available and reproducible. Field data is the verdict; lab data is the tool for finding problems and verifying fixes.
The WebYes scan measures lab values on the scanned pages and includes them in the speed pillar. To learn how to run and interpret that measurement yourself, also read testing website speed.
The most common causes of poor scores
A high LCP is usually caused by oversized images, slow server response or render-blocking scripts and stylesheets. A high INP almost always points to too much JavaScript occupying the main thread, often from tag managers, chat widgets and tracking scripts. CLS problems come from images without fixed dimensions, late-loading ads or web fonts that make the text jump.
The order of attack is almost always: optimise images first (size, compression, lazy loading), then remove or defer unnecessary scripts, and finally speed up the server side with caching or a CDN.
Related articles
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Frequently asked questions
Are Core Web Vitals an official ranking factor?
Yes. Google has confirmed that the Core Web Vitals are part of the page experience signals that count towards search rankings. The effect is biggest when competing pages have comparable content: then speed tips the balance.
What happened to First Input Delay (FID)?
FID was replaced by INP in March 2024. INP measures the full response time of all interactions on a page instead of only the delay of the first one. That makes it a stricter and more realistic measure of how slow a page feels.
Does the WebYes scan measure Core Web Vitals?
Yes. Speed is one of the four pillars of the scan. Among other things we measure LCP, CLS and the overall loading performance of the scanned pages, and show per finding where the delay comes from.
The WebYes scan measures this too
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