Core Web Vitals are a set of specific metrics Google uses to evaluate the user experience of web pages. Since 2021, they have been a direct ranking factor in Google Search. Understanding and improving them is essential for both performance and SEO.

The Three Core Web Vitals

Largest Contentful Paint (LCP)

LCP measures loading performance — specifically, when the largest visible content element in the viewport finishes rendering. Google considers LCP good when it occurs within 2.5 seconds of page load.

Common LCP elements:

Hero images
Large text blocks (H1s with no image above)
Background images with content on top

How to improve LCP:

Optimize and compress hero images (use WebP/AVIF format)
Use a Content Delivery Network (CDN)
Preload critical resources with <link rel="preload">
Remove render-blocking JavaScript and CSS
Implement server-side rendering or static generation

Cumulative Layout Shift (CLS)

CLS measures visual stability — how much the page layout shifts unexpectedly as it loads. A good CLS score is 0.1 or less.

Common causes of layout shifts:

Images without explicit width/height attributes
Ads, embeds, or iframes with no reserved space
Dynamically injected content above existing content
Web fonts causing FOIT (Flash of Invisible Text) or FOUT

How to improve CLS:

Always set width and height on images and video elements
Reserve space for ads and dynamic content
Use font-display: optional for web fonts
Avoid inserting content above existing content unless triggered by user interaction

Interaction to Next Paint (INP)

INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. It measures interactivity — the latency of all user interactions (clicks, taps, keyboard input) throughout the page lifecycle. A good INP is 200ms or less.

How to improve INP:

Break up long tasks (tasks taking >50ms on the main thread)
Defer non-essential JavaScript
Use web workers for heavy computation
Optimize event handlers to complete quickly
Minimize third-party script impact

How to Measure Core Web Vitals

Field Data vs Lab Data

Field data (Real User Monitoring) comes from actual users via the Chrome User Experience Report (CrUX). This is what Google uses for ranking. Use Google Search Console → Core Web Vitals report.

Lab data comes from controlled tests like Lighthouse, PageSpeed Insights, or Krokanti Audit. Lab data is useful for debugging but may differ from field data.

Tools

Krokanti Audit: Run a full audit to see all three CWVs with pass/fail ratings and specific recommendations
Google PageSpeed Insights: Shows both lab and field data
Chrome DevTools: Performance tab for detailed profiling
Web Vitals Chrome Extension: Real-time overlay while browsing

The Business Impact

Research consistently shows that improving Core Web Vitals leads to measurable business outcomes:

Every 100ms improvement in LCP correlates with a ~1% increase in conversion rate (Google/Deloitte)
Sites with good CLS had 24% fewer session abandonment rates (Google)
Improving INP can reduce bounce rates by 8-20% depending on the industry

A Practical Checklist

Before your next deployment, verify:

All images have explicit width and height attributes
Hero images are preloaded and served in modern formats
No render-blocking scripts in <head> without defer or async
Web fonts use font-display: swap or optional
No unexpected layout shifts when scrolling or interacting
All click/tap handlers respond in under 200ms

Run a free audit on your site with Krokanti Audit to see your exact Core Web Vitals scores and get specific recommendations tailored to your site.