Core Web Vitals

Largest Contentful Paint measures when the largest element in the viewport finished rendering. Good is 2.5 seconds or less; poor is above 4 seconds.

Cumulative Layout Shift measures how much visible content moved unexpectedly during the page's lifetime. Good is 0.1 or less; poor is above 0.25.

Interaction to Next Paint measures the latency of interactions across the whole visit, reporting close to the worst one. Good is 200 milliseconds or less; poor is above 500.

Interaction to Next Paint replaced First Input Delay in 2024, because First Input Delay measured only the delay before processing began on the first interaction and almost every page passed it.

Field data comes from real visits — real devices, real networks, real interactions — and is what the assessment uses. The Chrome User Experience Report is its public form.

Lab data comes from a simulated load in a controlled environment. It is reproducible, available immediately, and attributes a problem to a specific resource.

They disagree routinely, and neither is wrong. A laboratory test on a fast connection will not reproduce what a mid-range phone on a congested network experiences.

Some metrics cannot be measured in a laboratory at all: Interaction to Next Paint requires someone to interact, so a synthetic load reports nothing for it.

Each metric is evaluated at the 75th percentile of visits over a rolling 28-day window.

A URL passes only if all three metrics are in the good range. One poor metric fails the assessment.

Data is grouped by URL where there is enough of it, and falls back to a group of similar URLs otherwise, which is why a low-traffic page shows its section's data rather than its own.

Because the window is 28 days, an improvement shipped today appears gradually over the following month rather than immediately.

Google describes page experience, which includes these metrics, as one of many signals, and states that content relevance remains far more important.

The honest framing is that these metrics describe something worth improving for its own sake — visitors abandon slow and unstable pages — and that the search effect is real but modest.

Search Console's Core Web Vitals report groups URLs by status and is the authoritative view of how Google assesses a site.

PageSpeed Insights shows both field data, when it exists for the URL, and a laboratory run, side by side, which is the fastest way to see whether they agree.