Core Web Vitals: What You Need to Know

What are Core Web Vitals

Core Web Vitals (CWV) is a subset of Page Experience signals used by Google for ranking. Unlike Lighthouse lab tests, CWV are measured on real users via Chrome User Experience Report (CrUX).

The three current CWV metrics:

LCP — Largest Contentful Paint

LCP measures the time until the largest visible element on the page is displayed — usually a hero image or H1.

Value	Rating
≤ 2.5 seconds	✅ Good
2.5 – 4.0 seconds	⚠️ Needs improvement
> 4.0 seconds	❌ Poor

How to improve LCP

• Optimize images — convert to WebP or AVIF, add loading="eager" for hero images.
• Preload LCP element — add <link rel="preload" as="image">.
• Use CDN — reduces TTFB.
• Remove blocking resources — move non-critical JS and CSS to defer/async.
• Server-Side Rendering — SSR drastically improves LCP for SPA.

CLS — Cumulative Layout Shift

CLS measures the cumulative shift of page elements during loading.

Value	Rating
≤ 0.1	✅ Good
0.1 – 0.25	⚠️ Needs improvement
> 0.25	❌ Poor

How to improve CLS

• Specify image sizes — always add width and height.
• Reserve space for ads — set fixed sizes for ad blocks.
• Do not insert content above existing — banners and cookie notices should be at the bottom.
• Preload fonts — FOUT causes shifts.

INP — Interaction to Next Paint

INP replaced FID in March 2024. It measures the delay between user interaction and screen update.

Unlike FID, INP tracks all interactions and takes the worst value.

Value	Rating
≤ 200 ms	✅ Good
200 – 500 ms	⚠️ Needs improvement
> 500 ms	❌ Poor

How to improve INP

• Break up long tasks — any JS > 50ms blocks the browser.
• Reduce JS bundle size — code splitting and tree shaking.
• Avoid synchronous operations in event handlers.
• Check third-party scripts — chat bots and analytics often cause poor INP.

How to Measure Core Web Vitals

Field Data

• Google Search Console → "Page Experience".
• PageSpeed Insights — combines Field and Lab Data.
• CrUX Dashboard in Looker Studio.

Lab Data

• Lighthouse in Chrome DevTools.
• WebPageTest — detailed waterfall testing.

Impact of CWV on SEO

Google confirmed: CWV is a ranking factor, but content is still more important.

Practically: if your site is in the green zone, CWV won't hinder growth.