How it works
Field data and lab data
The check asks Google's PageSpeed Insights API, which returns two things. Field data comes from the Chrome User Experience Report: the 75th percentile of real Chrome visits over the last 28 days, for the exact page when it has enough traffic, otherwise for the whole site (origin), and nothing when neither has enough.
Lab data comes from one Lighthouse run in Google's data centre on a simulated phone or desktop. It gives a 0–100 performance score, lab timings and the opportunities and diagnostics Lighthouse flagged, worded in the page's language. A lab run has no real user input, so it cannot measure INP; Total Blocking Time is its closest stand-in.
The thresholds
Each field metric is rated with Google's published thresholds (web.dev): good, needs improvement, or poor. The page passes the Core Web Vitals assessment only when LCP, INP and CLS are all present and all good.
LCP good ≤ 2.5 s, poor > 4 s · INP good ≤ 200 ms, poor > 500 ms · CLS good ≤ 0.1, poor > 0.25 · FCP good ≤ 1.8 s · TTFB good ≤ 0.8 s
