CrUX field data vs lab data

Whether these figures come from real Chrome users on this site over the last 28 days, or from a single lab run standing in when there is too little traffic to report on.

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Every Core Web Vitals figure comes from one of two very different places, and it matters which one you are reading.

Field data (CrUX)

The Chrome UX Report collects timings from real Chrome users who opted in to share usage statistics. PageSpeed Insights publishes its 75th percentile over a rolling 28-day window — the value three quarters of visits beat. This is the data Google uses for search, and the only source of INP. SiteScan shows it under "Real users, last 28 days (CrUX)". When the home page itself has too few visits, it falls back to figures for the whole origin, and the heading says "whole site".

Lab data (Lighthouse)

A single synthetic run of the home page from Google's servers, with a simulated device and connection. It is repeatable and available for any site, but it does not know your audience's devices, networks or interactions, and it varies a little between runs. These are the rows at the top of the panel, including the performance score.

No field data? Small and new sites often have too little Chrome traffic for the report. SiteScan then says so, and the lab rows are all there is. That is not a fault; it just means the numbers are an estimate. Field data catches up within 28 days of traffic growing. Read more in Core Web Vitals.