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Core Web Vitals

Core Web Vitals — Google's standard set of user-experience metrics: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS).

Also known as: web vitals, LCP, INP, CLS

Definition

Core Web Vitals are Google’s three headline UX metrics for the web — Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift — measured at the 75th percentile over a rolling window.

In Maxoperf

Maxoperf’s monitoring roadmap includes browser and page-speed signals alongside protocol-level checks. Today, treat Core Web Vitals as a complementary signal: load tests show how systems behave under pressure, while browser metrics show what users actually experienced.

Common pitfalls

  • Optimising for the lab Lighthouse score and ignoring real-user p75 — the two diverge once you have meaningful international traffic.
  • Treating CLS as a one-time fix — late-loading ads or auth banners often regress it after months of stability.

FAQ

Why measure Web Vitals from real users instead of synthetic checks?

Web Vitals depend on the user device, network, and browser version — synthetic checks give a baseline; RUM tells you what your real users actually experienced.

What are the recommended thresholds?

Per Google: LCP under 2.5 s, INP under 200 ms, and CLS under 0.1 are the usual good thresholds at the 75th percentile.