PerfoMetrix

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Core Web Vitals Checker

Measure LCP, CLS, INP and the Lighthouse performance score for any URL, with real-user Chrome data when your page has enough traffic.

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What the Core Web Vitals Checker checks

Core Web Vitals are Google’s three metrics for how a page feels to real visitors: Largest Contentful Paint (LCP) for loading, Interaction to Next Paint (INP) for responsiveness, and Cumulative Layout Shift (CLS) for visual stability. A page passes when, at the 75th percentile of visits, LCP is 2.5 seconds or less, INP is 200 milliseconds or less and CLS is 0.1 or less.

This checker runs Google PageSpeed Insights on the mobile strategy. You get the Lighthouse performance score and lab metrics (LCP, CLS, Total Blocking Time, First Contentful Paint and Speed Index) from a simulated mid-range phone. When the page, or failing that the whole site, has enough Chrome traffic, you also get real-user field data from the Chrome UX Report for LCP, INP and CLS. A run usually takes 20 to 40 seconds.

Why it matters

Field data is what Google uses for the page experience signals in Search, and it reflects your actual visitors, their devices and networks. Lab data is a controlled test that helps explain why the field numbers look the way they do. INP, which replaced First Input Delay in March 2024, has no lab equivalent, so Total Blocking Time is the closest lab proxy.

Speed is also simply a usability issue. Slow, jumpy pages get abandoned, particularly on mobile connections, whatever their ranking.

How to fix common problems

For LCP, find the LCP element (often a hero image or heading) and make it discoverable early: serve the image in the HTML rather than via JavaScript, add fetchpriority="high", never lazy-load it, compress it to WebP or AVIF, and cut server response time with caching or a CDN.

For CLS, give every image, video, iframe and ad slot explicit width and height or an aspect-ratio, reserve space for banners and embeds, and use font-display: swap with size-matched fallback fonts.

For INP, reduce JavaScript on the main thread: remove unused third-party scripts, split long tasks, defer non-critical code and keep event handlers short. Field data updates over a rolling 28-day window, so improvements take weeks to show. The full PerfoMetrix audit adds security, SEO and AI-readiness results to these metrics, 170+ checks in one report.

Questions

Why are my lab and field numbers different?

Lab data comes from one simulated device and network. Field data is the 75th percentile of real Chrome visits over 28 days. Real visitors use faster and slower devices, warm caches and interact with the page, so the two rarely match.

Why is there no field data for my page?

The Chrome UX Report only publishes data for pages and origins with enough eligible traffic. When the page has too little, the tool shows site-wide data if available, and otherwise only the lab results.

Is the Lighthouse performance score a ranking factor?

No. Google uses field Core Web Vitals, not the Lighthouse score. The score is a useful lab summary for spotting regressions and opportunities.

Why does the score change between runs?

Lab tests vary with server load, network conditions and third-party scripts. Small swings are normal; look at trends and at the underlying metrics rather than a single number.