What are Core Web Vitals?
Core Web Vitals are Google's set of page experience metrics used as a ranking signal. The three metrics are: LCP (Largest Contentful Paint) — how long it takes for the main content to load, INP (Interaction to Next Paint) — how responsive the page is to user interactions, and CLS (Cumulative Layout Shift) — how stable the layout is as the page loads.
Core Web Vitals thresholds and field data
Google's target thresholds: LCP under 2.5 seconds (good), under 4 seconds (needs improvement), over 4 seconds (poor). INP under 200ms (good). CLS under 0.1 (good). Pages passing all three are considered to have a good page experience.
Core Web Vitals are measured using real-user data from Chrome (CrUX data), not just lab-based testing tools. A page can score well in PageSpeed Insights but still fail in the field if real users experience slower loads due to network conditions or device capabilities.
Example
Example
A page with a hero image that is not properly preloaded may have an LCP of 5+ seconds. Fixing the image delivery typically cuts LCP by 40-60% on mobile.
Frequently asked questions
What are the three Core Web Vitals metrics?
LCP (Largest Contentful Paint) for loading speed, INP (Interaction to Next Paint) for responsiveness, and CLS (Cumulative Layout Shift) for visual stability. Good thresholds: LCP under 2.5s, INP under 200ms, CLS under 0.1.
Why does my page pass lab tests but fail Core Web Vitals?
Google scores field data from real Chrome users (CrUX), not lab runs. Slower devices and networks in your actual audience can fail a page that looks perfect in PageSpeed Insights.