Core Web Vitals are a set of standardized field metrics from Google that measure real user experience: LCP for loading speed, INP for interactivity, and CLS for visual stability. INP replaced FID as the responsiveness metric in March 2024. Targets are LCP under 2.5 s, INP under 200 ms, and CLS under 0.1, measured at the 75th percentile of page loads.
Why it matters
Core Web Vitals are part of Google's page-experience signals and influence ranking on competitive queries. More importantly, they reflect what users actually feel: how fast the page paints, how quickly it responds, and whether content jumps around. Poor scores correlate with higher bounce rates and lower conversion, so improving them tends to pay back in both SEO and revenue.
How to measure
- Use RUM (the Chrome User Experience Report or your own analytics) for field data — that is what Google ranks on.
- Run Lighthouse or PageSpeed Insights for lab diagnostics that point to specific fixes.
- Track TTFB and total blocking time alongside the headline metrics to catch root causes early.
- Optimize the LCP element first: serve a properly sized image, preload it, and avoid render-blocking CSS or JS.
- Reduce long tasks and break up JavaScript bundles to improve INP.
- Reserve space for media and embeds to keep CLS stable. Web.dev maintains current thresholds in its Web Vitals guide.