Web Vitals (web.dev overview)

Google web.dev’s canonical overview of Web Vitals — the source page behind the core-web-vitals concept. Pins the thresholds and the lab-vs-field framing to a Google source.

The three Core Web Vitals + thresholds

• LCP (loading) — should occur within 2.5 seconds of when the page first starts loading.
• INP (interactivity) — pages should have an INP of 200 milliseconds or less.
• CLS (visual stability) — pages should maintain a CLS of 0.1 or less.

The recommended judging point is the 75th percentile of page loads, “segmented across mobile and desktop devices” — i.e. a page passes only when most real users, on both form factors, hit “good.”

Lab vs field — and the supporting vitals

web.dev frames the split this wiki leans on: lab measurement is “the best way to test the performance of features during development—before they’ve been released to users,” while field measurement matters because real performance “can substantially vary based on a user’s device capabilities, their network conditions, what other processes may be running.”

It names two supporting metrics that aren’t Core Web Vitals but help diagnose them: TTFB (Time to First Byte) and FCP (First Contentful Paint), both “vital aspects of the loading experience… useful in diagnosing issues with LCP.” This formalizes the wiki’s “diagnose with the lab, judge with the field” chain: TTFB/FCP are upstream diagnostics, LCP is the field outcome.

Why this matters here

Gives core-web-vitals a primary Google citation for its thresholds and the p75 rule, and adds the TTFB/FCP supporting-metric layer the concept page lacked — the lab-side diagnostics that sit before crux‘s field judgment. See lighthouse (lab) vs crux (field) and synthesis.