Core Web Vitals
Core Web Vitals are the three metrics Google uses to measure page experience: LCP (how quickly the main content loads), INP (how responsive the page is to interaction), and CLS (visual stability). Since 2024, INP has replaced FID.
The thresholds
| Metric | Meaning | Good | Needs improvement | Poor |
|---|---|---|---|---|
| LCP | Time until the largest element is visible | ≤ 2.5 s | 2.5–4 s | > 4 s |
| INP | Response to clicks and typing | ≤ 200 ms | 200–500 ms | > 500 ms |
| CLS | Unexpected layout shifts | ≤ 0.1 | 0.1–0.25 | > 0.25 |
These numbers come from field data (CrUX), not only the lab. PageSpeed Insights can differ from real user experience because of device, network, and location.
Common causes
- Slow LCP: oversized hero images, high TTFB, render-blocking CSS/JS, slow fonts.
- Poor INP: heavy JavaScript, long event handlers, third-party scripts, slow hydration.
- High CLS: images without dimensions, injected ads/embeds, font swaps that move text.
Fix without guessing
Measure first in PageSpeed Insights and Search Console → Core Web Vitals. Fix the largest cause, then measure again after cache and CDN behavior is consistent. For LCP, check TTFB and web cache. For assets, compression and a CDN often help.
What to watch for
- Field data wins. Good lab data does not guarantee good field data.
- Mobile is stricter. Test on realistic devices and networks.
- Third-party scripts are expensive. Every analytics or chat tag adds JavaScript cost.
- Do not chase a score of 100. The thresholds matter, not a cosmetic number.
FAQ
Are Core Web Vitals a ranking factor?
Google uses page experience as one signal, but relevance and content quality remain far more important.
Why is lab data good but field data poor?
Lab tests usually use fast devices and networks. Field data reflects real visitors with different devices, locations, and connections.
Which metric should I fix first?
Start with LCP if the page feels slow, then INP if interactions lag. CLS is usually the cheapest to fix by setting image dimensions.
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