Core Web Vitals are three measurements Google uses to judge how a page feels to real visitors: how fast the main content appears, how quickly the page responds when you tap something, and whether things jump around while loading. They're a ranking signal, but more importantly, they affect whether visitors stay.
The three metrics
| Metric | What it measures | Good | Poor |
|---|---|---|---|
| LCP (Largest Contentful Paint) | How long until the main content, usually the hero image or heading, appears | 2.5 s or less | Over 4 s |
| INP (Interaction to Next Paint) | How quickly the page responds to taps, clicks and typing | 200 ms or less | Over 500 ms |
| CLS (Cumulative Layout Shift) | How much the layout jumps while loading | 0.1 or less | Over 0.25 |
INP replaced the older First Input Delay (FID) metric in March 2024. It's stricter, because it measures every interaction, not just the first.
Lab data vs field data
- Field data comes from real Chrome users visiting your site, collected in the Chrome UX Report (CrUX). This is what Google uses for rankings.
- Lab data comes from a simulated test, such as Lighthouse. Useful for debugging, but not the same as real users.
PageSpeed Insights shows both. If your site doesn't have enough traffic, you'll only see lab data.
For Sri Lankan sites, field data often looks worse than lab tests from a fast office connection, because many real visitors use mid-range phones on mobile data.
How to fix a slow LCP
- Serve a properly sized, compressed hero image in WebP or AVIF
- Don't lazy-load the main above-the-fold image
- Preload the LCP image or font if needed
- Reduce server response time with better hosting, caching and a CDN
- Remove render-blocking CSS and JavaScript where possible
How to fix a poor INP
INP problems usually come from too much JavaScript:
- Remove unused plugins and scripts, such as old chat widgets, sliders and trackers
- Delay non-essential third-party scripts until after the page loads
- Break up long JavaScript tasks
- Avoid heavy page builders that ship large scripts on every page
How to fix CLS
- Set width and height on images and videos
- Reserve space for ads, banners and embeds
- Avoid inserting content above existing content after load, such as cookie bars that push the page down
- Use font-display: swap with similar fallback fonts
How much do they matter for rankings?
Core Web Vitals are a real but modest ranking factor. Great content on a slow site can still outrank thin content on a fast one. But when competing pages are similar, speed can be the tie-breaker, and speed always affects conversions.
Check your site
- Open PageSpeed Insights and test your key pages on mobile.
- In Search Console, open the Core Web Vitals report to see which URL groups are poor.
- Fix the biggest issues first, usually images and scripts.
Read website speed for Sri Lankan mobile users for practical fixes, or see our speed optimisation service.