Core Web Vitals Explained: A Practical Guide to LCP, INP, and CLS

Why these three metrics matter

Google uses Core Web Vitals as part of its page experience signal. They describe what real users experience in the field, not what a synthetic lab test reports, so improving them is genuine product work.

Largest Contentful Paint (LCP)

LCP measures when the largest above-the-fold element finishes loading. The target is under 2.5 seconds.

• Preload the hero image and the stylesheet that affects it.
• Serve images in modern formats with sensible dimensions.
• Cut render-blocking third-party scripts from the critical path.

Interaction to Next Paint (INP)

INP replaces First Input Delay. It measures the worst response to a user interaction, and the good limit is 200 milliseconds.

• Keep long tasks on the main thread under 50 milliseconds.
• Defer non-essential work until after the page becomes interactive.
• Debounce expensive handlers instead of running them on every event.

Cumulative Layout Shift (CLS)

CLS rewards pages that do not jump around while loading. A score below 0.1 is good.

• Reserve space for images and embeds with explicit dimensions.
• Avoid inserting content above existing content after the request starts.
• Let fonts keep their fallback metrics to stop invisible-text reflows.

How to verify your work

Run Lighthouse for a quick lab pass, then compare it to CrUX field data for your real traffic. Field performance is what search engines ultimately see, so an analytics tool that reports RUM metrics is worth the setup cost.

The takeaway

Performance is not a one-time checklist. Set the targets, add the metrics to your CI, and treat every release that regresses them as a bug.