The three Core Web Vitals
Largest Contentful Paint (LCP) is when the biggest image or text block on the first screen appears - good is 2.5 seconds or less, poor is over 4. Interaction to Next Paint (INP) is how quickly the page responds to taps and clicks - good is 200 ms or less, poor over 500 ms; it replaced First Input Delay in March 2024. Cumulative Layout Shift (CLS) measures how much content jumps around while loading - good is 0.1 or less, poor over 0.25. A page passes when 75% of real visits are good on all three.
Real users versus the lab
The verdict at the top comes from the Chrome UX Report: what real Chrome users experienced over the last 28 days. Sites without enough visitors have no such data, and INP can only be measured with real interactions. The lab section is a single test run on Google's machines - repeatable, and the place to find causes. In the lab, Total Blocking Time stands in for INP.
What makes this checker useful
It is the full PageSpeed Insights report, for mobile and desktop at once: the Performance, Accessibility, Best Practices and SEO scores, real-user data for the page and for the whole site, the lab metrics with a filmstrip of the page loading, and every failing audit with the scripts, images or elements behind it. Filter the performance issues by the metric they hurt (LCP, CLS, TBT or FCP), and download everything as an Excel workbook - one row per issue and per affected file - to share with a developer or tick off as you go. The test runs through Google's PageSpeed Insights, so the numbers match what Google itself reports.
Typical fixes
A slow LCP is most often a large hero image (serve it smaller and in AVIF or WebP - the image converter does both), an image only found after JavaScript runs, or render-blocking CSS. Layout shifts come from images and embeds without a width and height, and from banners injected above content. A slow server response shows up as a long first phase - check it worldwide with the TTFB test. For the full walkthrough, read our Core Web Vitals guide.
