Core Web Vitals - LCP, INP, CLS Explained

What are Core Web Vitals?

Core Web Vitals (CWV) are three metrics Google uses to measure real-world page experience: Largest Contentful Paint (LCP) for loading, Interaction to Next Paint (INP) for responsiveness, and Cumulative Layout Shift (CLS) for visual stability. Each metric has good, needs-improvement, and poor thresholds, and they are direct ranking signals in Google's algorithm.

Why Core Web Vitals Matters

Core Web Vitals affect both rankings and conversion rates. Google's own data shows sites that improve their CWV pass rate to 75% or higher see measurable lifts in organic traffic, and a 2024 study by Portent showed every additional second of LCP reduces conversion rates by an average of 4.42%. CWV is one of the few page experience metrics Google has confirmed as a ranking factor.

How Core Web Vitals Works

Google measures Core Web Vitals using anonymised field data from real Chrome users, aggregated in the Chrome User Experience Report (CrUX). The 75th percentile of your page's measurements determines whether each metric is rated good, needs improvement, or poor. You can also test in the lab using Lighthouse or PageSpeed Insights, but field data is what counts for ranking.

The three metrics each correspond to a different part of the loading experience. LCP fires when the largest visible element finishes painting. INP measures the worst input delay across the session. CLS measures the cumulative visual jumps that happen as the page loads in. Improving all three usually means tackling images, JavaScript execution, and reserved space for dynamic elements.

Core Web Vitals Thresholds

✓LCP (Largest Contentful Paint): good = under 2.5s, poor = over 4.0s

✓INP (Interaction to Next Paint): good = under 200ms, poor = over 500ms

✓CLS (Cumulative Layout Shift): good = under 0.1, poor = over 0.25

✓Field data via Chrome User Experience Report (CrUX) is the source of truth

✓Lab data via Lighthouse is useful for debugging but not for ranking

✓Measurement is taken at the 75th percentile of all user sessions

✓Page experience signal applies per URL, not per domain

How to Improve Core Web Vitals

Compress and serve hero images in AVIF or WebP with explicit width and height attributes

Preload the LCP element (hero image or first text block) to speed up initial render

Defer non-critical JavaScript and reduce main-thread work to improve INP

Reserve space for ads, embeds, and dynamic banners to prevent CLS

Use a CDN and HTTP/3 to cut server response times

Remove or replace heavy third-party scripts (chat widgets, analytics, A/B testing)

Measure with PageSpeed Insights and the CrUX dashboard every 4-6 weeks

Frequently Asked Questions

Are Core Web Vitals a ranking factor?

Yes. Google rolled Core Web Vitals into the page experience signal in 2021 and confirmed them as direct ranking factors. The effect is modest for most queries but becomes a meaningful tiebreaker between similarly relevant pages.

What's the difference between INP and FID?

FID (First Input Delay) measured only the first interaction. INP (Interaction to Next Paint) replaced FID in March 2024 and measures responsiveness across the entire session, which gives a much more representative view of real-world experience.

Where do I see my Core Web Vitals data?

Google Search Console has a Core Web Vitals report that uses field data from CrUX. PageSpeed Insights shows both lab and field data per URL. The CrUX dashboard gives historical trends, and DebugBear or SpeedCurve provide deeper RUM data for paid tracking.