Core Web Vitals are Google's official metrics for measuring real-world user experience on websites. They directly affect search rankings and tell you whether your pages feel fast and stable to visitors.
What are Core Web Vitals?
Google introduced Core Web Vitals as a standardized way to measure what users actually experience. Unlike traditional speed metrics that measure technical performance, these focus on perceived experience.
The metrics answer three questions:
- How fast does the main content load? (LCP)
- How quickly can users interact? (INP)
- Does the page shift around unexpectedly? ()
These metrics became a ranking factor in 2021. Sites that pass all three thresholds get a small ranking boost. More importantly, they correlate with better conversion rates and lower bounce rates.
The Three Metrics
Largest Contentful Paint (LCP)
Measures loading performance. LCP tracks when the largest visible element (usually hero image or heading) finishes rendering.
Thresholds:
- Good: under 2.5 seconds
- Needs improvement: 2.5-4 seconds
- Poor: over 4 seconds
Interaction to Next Paint (INP)
Measures responsiveness. INP replaced FID in 2024 and tracks how quickly the page responds to user interactions like clicks, taps, and key presses throughout the entire session.
Thresholds:
- Good: under 200 milliseconds
- Needs improvement: 200-500 milliseconds
- Poor: over 500 milliseconds
Cumulative Layout Shift (CLS)
Measures visual stability. CLS tracks how much visible content shifts unexpectedly during page load. Ads loading late, images without dimensions, and all cause layout shifts.
Thresholds:
- Good: under 0.1
- Needs improvement: 0.1-0.25
- Poor: over 0.25
How to Interpret Core Web Vitals
Google measures these from real user data (field data) and lab tests. Field data from Chrome User Experience Report (CrUX) shows what actual visitors experience. Lab data from Lighthouse shows potential issues in controlled conditions.
Field vs Lab differences are normal. Your server might be fast, but users on slow mobile connections see different results. Focus on field data for real impact; use lab data for debugging.
Check by page type. Homepage might pass while product pages fail. Category pages with many images might have different CLS than text-heavy blog posts.
Tools to monitor:
- Google Search Console (Core Web Vitals report)
- PageSpeed Insights (both field and lab data)
- Chrome DevTools (lab testing)
- Web Vitals JavaScript library (real-time monitoring)
