Core Web Vitals: a developer checklist
LCP, INP, and CLS in plain language—what to measure and where to look first.
Google’s Core Web Vitals summarize real-user experience in three metrics: loading (LCP), interactivity (INP), and visual stability (CLS). They are not the only signals that matter, but they are a practical starting point for performance work.
Largest Contentful Paint (LCP)
Measures when the main content becomes visible. Large hero images, slow fonts, and render-blocking scripts are common culprits. Optimize images, preload critical assets, and keep critical paths lean.
Interaction to Next Paint (INP)
Replaces FID as the responsiveness metric. Long tasks on the main thread hurt INP—split work, defer non-critical scripts, and avoid expensive layout thrashing after interactions.
Cumulative Layout Shift (CLS)
Tracks unexpected movement. Always set width and height (or aspect ratio) on media, reserve space for ads and embeds, and avoid inserting content above existing content without user intent.
Next steps
Use field data (CrUX, RUM) when possible; lab tools like Lighthouse complement but do not replace real users. Pick one metric in the red, fix the biggest lever, and measure again.