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Core Web Vitals are Google’s three main measures of how a page feels to use. They check how fast the main content appears, how quickly the page reacts when you click, and whether things jump around while it loads.
They matter for search, but they matter more for people. A slow, jumpy page loses visitors before they read a word.
The three Core Web Vitals
| Metric | What it measures | Good | Poor |
|---|---|---|---|
| Largest Contentful Paint (LCP) | How long the biggest visible element takes to show | 2.5s or less | Over 4s |
| Interaction to Next Paint (INP) | How fast the page responds to clicks, taps and key presses | 200ms or less | Over 500ms |
| Cumulative Layout Shift (CLS) | How much the layout moves unexpectedly | 0.1 or less | Over 0.25 |
Anything between good and poor counts as “needs improvement”.
Google measures each one at the 75th percentile. That means at least three out of four visits need to hit the good threshold, so a fast result on your own laptop isn’t enough.
INP replaced First Input Delay (FID) in March 2024. FID only measured the delay before the first interaction. INP looks at interactions across the whole visit, so it’s a stricter test.
Why Core Web Vitals matter for conversion
Each metric lines up with a moment where visitors give up:
- Slow LCP. The visitor stares at a blank or half-built page and hits back.
- Poor INP. They click “Book a demo” and nothing seems to happen, so they click again or leave.
- High CLS. A banner or image loads late, the page jumps, and they tap the wrong thing.
On a SaaS site the hero section is often the LCP element, since it holds the biggest image or headline above the fold. That makes the hero the first place to look when LCP is slow.
Field data vs lab data
Field data comes from real Chrome users through the Chrome UX Report. It’s what Google uses, and you can see it in PageSpeed Insights and the Core Web Vitals report in Search Console.
Lab data comes from a test run on one device, like Lighthouse. It’s useful for debugging, but it can’t measure INP directly, since no real person is clicking. Lighthouse uses Total Blocking Time as a stand-in.
If the two disagree, trust field data. Low-traffic pages may not have enough field data, in which case Google groups them with similar pages or the whole site.
How to improve each metric
LCP
- Serve the hero image in a modern format like WebP or AVIF, at the right size.
- Don’t lazy-load the LCP image. Preload it or give it a high fetch priority.
- Cut render-blocking scripts and fonts, and serve the page from a fast host or CDN.
INP
- Ship less JavaScript, especially third-party tags, chat widgets and trackers.
- Break long tasks into smaller ones so the browser can respond between them.
- Give instant visual feedback on clicks, even if the real work takes longer.
CLS
- Set width and height (or aspect ratio) on images, videos and embeds.
- Reserve space for cookie banners, announcement bars and late-loading content.
- Load web fonts in a way that avoids a big jump when they swap in.
Common Core Web Vitals mistakes
- Chasing a Lighthouse score of 100. The lab score is a tool, not the target. Field data is what counts.
- A hero video that autoplays at full size. It often becomes the LCP element and drags the whole page down.
- Piling on marketing scripts. Each tag seems small, but together they hurt INP.
- Testing only on a fast laptop. Most poor scores come from mid-range phones on mobile networks.