The three Core Web Vitals
Largest Contentful Paint (LCP) measures when the main content appears; aim for 2.5 seconds or less. Interaction to Next Paint (INP) measures how quickly the page responds to taps and clicks; aim for 200 milliseconds or less. INP replaced First Input Delay as a Core Web Vital in March 2024. Cumulative Layout Shift (CLS) measures how much content jumps while loading; aim for 0.1 or less.
Google evaluates each metric at the 75th percentile of real visits, so three quarters of your visitors need a good experience for a URL to pass.
Field data vs lab data
Field data comes from real Chrome users through the Chrome UX Report and is what Google uses in ranking. Lab data comes from a single Lighthouse test on a simulated mid-range phone and is useful for debugging. Small or new sites often have no field data yet, so this checker falls back to lab results.
Fixing slow pages
Most LCP problems come from large hero images, render-blocking CSS and JavaScript, or a slow server. INP problems usually come from long JavaScript tasks and heavy third-party scripts. CLS is usually caused by images and embeds without reserved space. Our blog guides on improving LCP and understanding CLS walk through each fix.
Frequently asked questions
Why does my score change between tests?
Lab tests vary with network and server conditions. Field data is a 28-day rolling average, so it moves slowly and is the more reliable signal.
Is TTFB a Core Web Vital?
No, but Time to First Byte is the floor under every loading metric. A slow server response makes a good LCP very hard to reach.
Does this check every page?
It tests your homepage. A full audit measures several page templates and crawls the site for the causes behind slow metrics.