Core Web Vitals became an official Google ranking factor in 2021, but many sites still score poorly on them. A good score isn't just about SEO — it directly correlates with user engagement, conversion rates, and bounce rate. Here's everything you need to know.
Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visible content element — usually a hero image or a headline — to fully render on screen. Google's threshold for 'Good' is under 2.5 seconds. The most common culprits for slow LCP are unoptimised images, render-blocking resources, and slow server response times.
The fastest fix: compress and convert hero images to WebP or AVIF, serve them from a CDN, and add `fetchpriority='high'` to the LCP element so the browser prioritises it in the resource waterfall.
Photo by Andrew Kliatskyi on Unsplash.
Interaction to Next Paint (INP)
INP replaced First Input Delay (FID) in 2024 as the responsiveness metric. It measures the latency of all user interactions across the page — clicks, taps, keyboard input — and reports the worst-case result. A score under 200ms is 'Good'; anything above 500ms needs urgent attention.
Heavy JavaScript main-thread work is the primary cause of poor INP. Audit with Chrome DevTools's Performance panel, break up long tasks with `scheduler.yield()`, and defer non-critical scripts to after the first interaction.
Cumulative Layout Shift (CLS)
CLS quantifies visual instability. A score of 0.1 or less is Good. Unexpected shifts are almost always caused by images without explicit dimensions, dynamically injected content (ads, cookie banners), or web fonts triggering a flash of unstyled text. Always specify `width` and `height` on all media elements — it's the single highest-impact CLS fix available.
Measuring and Monitoring
Use Google Search Console's Core Web Vitals report for field data (real users), and PageSpeed Insights for lab data (simulated). Remember: lab data is useful for debugging, but only field data from real users counts toward your ranking signal. Set up continuous monitoring with a tool like SpeedCurve or RUM to catch regressions before they impact rankings.