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Core Web Vitals explained

By Nalyzed · Published 2026-10-07 · Updated 2026-10-07 · 6 min read

Core Web Vitals are three field metrics Google uses to describe page experience: Largest Contentful Paint (loading, good at 2.5s or less), Interaction to Next Paint (responsiveness, good at 200ms or less) and Cumulative Layout Shift (visual stability, good at 0.1 or less). They are assessed at the 75th percentile of real visits, which is why a fast test on your own machine can coexist with a failing assessment.

The three metrics

All three are assessed at the 75th percentile across real visits, segmented by mobile and desktop. A page passes only when all three are in the good band for that segment.

  • Largest Contentful Paint (LCP) — when the largest visible element in the viewport finishes rendering. Good: 2.5s or less. Poor: over 4s.
  • Interaction to Next Paint (INP) — the worst-case latency between a user interaction and the next visual update, across the whole visit. Good: 200ms or less. Poor: over 500ms.
  • Cumulative Layout Shift (CLS) — how much visible content moves unexpectedly during the visit. Good: 0.1 or less. Poor: over 0.25.

Why lab and field numbers disagree

A Lighthouse run is a single simulated load on throttled hardware from one location. Field data is millions of real loads on real devices and networks. They answer different questions, and neither is wrong.

Lab data tells you what is slow and why, with a trace you can act on. Field data tells you whether your actual users experience it. Use lab data to diagnose and field data to decide whether you have a problem.

INP in particular cannot be measured properly in a lab run, because it depends on what a user does. A lab tool reports Total Blocking Time instead, which correlates with INP but is not the same metric.

What usually causes each failure

  • LCP: a large unoptimized hero image, a render-blocking stylesheet or font, a slow server response, or the LCP element being client-rendered so it cannot start loading until JavaScript has run.
  • INP: long JavaScript tasks on the main thread — hydration, large third-party scripts, expensive event handlers, or a heavy re-render on every keystroke.
  • CLS: images and ads without reserved dimensions, web fonts swapping in at a different size, content injected above existing content, or an animation on a layout-affecting property.

What to fix first

  1. Fix CLS first. It is usually the cheapest — explicit width and height on every image and embed, reserved space for anything injected, and font-display with a matched fallback metric.
  2. Then LCP. Identify the LCP element, make sure it is in the server-rendered HTML, preload it, serve it in a modern format at the right size, and remove render-blocking resources ahead of it.
  3. Then INP. Break up long tasks, defer or remove third-party scripts, and avoid synchronous work in frequently fired handlers.
  4. Re-measure in the field, not the lab. Field data moves on a 28-day rolling window, so a fix takes weeks to show fully.

How much do they matter for ranking?

Core Web Vitals are a real but small ranking input, and they are a tiebreaker rather than a lever: a fast page with a weak answer does not outrank a slow page with the right one. The stronger argument for fixing them is that layout shift and input delay cost conversions directly.

They also matter for GEO: an LCP element that only exists after JavaScript runs is frequently invisible to crawlers that do not execute it, so the same fix helps both.

Frequently asked questions

Why does PageSpeed Insights show two different scores?
The top section is field data from real Chrome users over the last 28 days; the lower section is a fresh Lighthouse lab run. The lab run is a diagnostic, the field data is the assessment.
What replaced First Input Delay?
Interaction to Next Paint (INP) replaced FID as a Core Web Vital in March 2024. INP measures the full latency to the next paint across all interactions, where FID only measured the delay before the first one began processing.
Do I need to pass all three metrics?
To get a passing Core Web Vitals assessment, yes — all three must be in the good band at the 75th percentile. Individual metrics still carry value on their own for user experience.
How long until a fix shows up in field data?
Field data uses a 28-day rolling window, so improvements appear gradually and take roughly a month to be fully reflected. Lab runs show the effect immediately.

Check your own site

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Published by Nalyzed. Scoring rules and limits are documented on the methodology page.