Core Web Vitals (CWV) is the set of real-user performance metrics Google uses as a ranking signal. The set was updated in March 2024: INP replaced FID. The current three are:
The three metrics
Largest Contentful Paint (LCP)
Time from navigation start until the largest visible element finishes rendering. Measures loading.
- Good: ≤ 2.5 seconds
- Needs improvement: 2.5 - 4 seconds
- Poor: > 4 seconds
Interaction to Next Paint (INP)
Worst-case time between any user interaction (click, tap, key press) and the next visual response. Measures interactivity. Replaced FID in March 2024.
- Good: ≤ 200 milliseconds
- Needs improvement: 200 - 500ms
- Poor: > 500ms
Cumulative Layout Shift (CLS)
Sum of unexpected layout shifts during the page's lifetime. Measures visual stability. Unitless score.
- Good: ≤ 0.1
- Needs improvement: 0.1 - 0.25
- Poor: > 0.25
Field vs lab data
Two ways to measure each:
- Field data: real users hitting your site, collected by Chrome (CrUX report). What Google actually uses for ranking. Available in Search Console and PageSpeed Insights.
- Lab data: a single synthetic run, via Lighthouse or PageSpeed Insights. Faster feedback but not what Google ranks on.
Lab data is good for catching regressions in development. Field data is what counts for SEO.
Common causes
| Bad metric | Most common cause |
|---|---|
| LCP > 2.5s | Large hero image not pre-loaded, slow server response, render-blocking JS |
| INP > 200ms | Heavy main-thread JS (React hydration, third-party tags) |
| CLS > 0.1 | Images without explicit width/height, ads that inject above-the-fold, late-loading web fonts shifting layout |
How much does it move rankings?
Google has been deliberately vague. Public statements: CWV is one of "many" signals; it's a tiebreaker more than a primary ranker. Anecdotal data from sites moving from "poor" to "good" CWV shows mid-single-digit traffic gains, not double-digit.
Practical takeaway: don't sacrifice content quality to chase CWV scores. But on pages where CWV is genuinely bad (LCP > 4s, INP > 500ms), the fixes pay back in CTR (faster pages get clicked more) more than in rankings (the signal is small).
Drop a Lighthouse JSON into the Report Builder to see your scores alongside other audit data.