Core Web Vitals

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Core Web Vitals are a set of real-world performance metrics Google uses as ranking signals. They measure three dimensions of page experience: loading performance, interactivity, and visual stability.

The three Core Web Vitals

Largest Contentful Paint (LCP)

LCP measures how long it takes for the largest visible element on the page to render. This is typically a hero image, a featured photograph, or a large block of text. It is Google’s primary measure of perceived load speed.

Good: under 2.5 seconds
Needs improvement: 2.5 to 4.0 seconds
Poor: over 4.0 seconds

The most common LCP issues are slow server response times, render-blocking resources (large JavaScript or CSS files that delay rendering), and unoptimised images loaded without compression or lazy loading.

Interaction to Next Paint (INP)

INP measures the responsiveness of a page to all user interactions (clicks, taps, keyboard inputs) throughout the session, including post-load interactions. It replaced First Input Delay (FID) as a Core Web Vital in March 2024.

Good: under 200ms
Needs improvement: 200 to 500ms
Poor: over 500ms

High INP is usually caused by heavy JavaScript execution that blocks the main thread. Third-party scripts (analytics, chat widgets, ad code) are frequent culprits.

Cumulative Layout Shift (CLS)

CLS measures unexpected layout shifts: elements moving around as the page loads. A button that jumps just as you tap it, or text that reflows when an image loads, are classic CLS problems.

Good: under 0.1
Needs improvement: 0.1 to 0.25
Poor: over 0.25

The most common causes are images without defined dimensions, dynamically injected content above existing content, and web fonts causing FOIT (Flash of Invisible Text) or FOUT (Flash of Unstyled Text).

How Google measures Core Web Vitals

Core Web Vitals are assessed using field data: real user experiences captured via the Chrome User Experience Report (CrUX), not lab simulations. Your scores are based on actual visits from real users over the previous 28 days.

Lab tools like Google PageSpeed Insights and Lighthouse show what performance looks like under controlled conditions, which is useful for diagnosis. They do not directly represent the field scores Google uses for ranking. If lab and field data disagree, the field data is what matters for SEO.

Where to check your Core Web Vitals

  • Google Search Console > Core Web Vitals report: shows field data segmented by URL group, with pass/fail status
  • PageSpeed Insights: combines field data (where available) with lab diagnostics and improvement recommendations
  • Chrome DevTools > Performance panel: detailed trace for diagnosing specific issues
  • Web Vitals Chrome extension: real-time field data as you browse your own site

Fixing common issues

Slow LCP:

  1. Improve server response time (TTFB). Consider CDN, caching, or upgrading hosting
  2. Preload the LCP element using <link rel="preload">
  3. Compress and correctly size images; use modern formats (WebP, AVIF)
  4. Defer non-critical JavaScript and CSS

High INP:

  1. Audit third-party scripts. Load them asynchronously or defer them
  2. Break up long tasks (over 50ms) using setTimeout or scheduler.yield()
  3. Reduce JavaScript bundle size; remove unused code

High CLS:

  1. Set explicit width and height on images and video
  2. Reserve space for dynamic content (ads, embeds) with CSS aspect ratios
  3. Preload web fonts; use font-display: optional to avoid FOIT

Core Web Vitals as a ranking factor

Google confirmed Core Web Vitals as a ranking signal with the Page Experience update in 2021. Their direct ranking impact is modest compared to relevance and authority signals. Passing the thresholds will not rescue poor content, and failing them will not sink excellent content.

Where they matter most is in close-competition scenarios. When two pages are roughly comparable in relevance and authority, page experience signals can determine which one ranks higher. For sites with large volumes of pages (news publishers, e-commerce, documentation), marginal improvements across thousands of URLs compound into meaningful ranking gains.

Frequently asked questions

Do Core Web Vitals affect all pages equally? No. Google requires sufficient field data to assess a URL. Pages with low traffic may fall back to domain-level scores, or may not be assessed individually at all.

Can I improve my scores without a developer? Some wins are accessible without code changes: compressing images, removing unused plugins on CMS platforms, switching to a faster hosting tier. The most impactful fixes (JavaScript optimisation, server-side improvements, font loading strategy) typically require developer involvement.

Do Core Web Vitals matter for mobile rankings? Yes. Google uses mobile-first indexing, so field data from mobile users primarily informs the score. Desktop and mobile scores are tracked separately in Search Console.