Core Web Vitals are a set of specific metrics that Google considers essential for measuring user experience on the web. These metrics focus on three key aspects of user interaction: loading performance, interactivity, and visual stability. Our website speed test tool measures these vital metrics and provides clear, actionable feedback to help you optimize your website's performance.
First Contentful Paint (FCP)
First Contentful Paint measures the time from when the page starts loading to when any part of the page's content is rendered on the screen. This includes text, images, SVGs, or canvas elements. FCP is important because it gives users the first visual signal that the page is actually loading — without it, users may think the page is broken or unresponsive. Google considers an FCP of 1.8 seconds or less as "good," between 1.8 and 3.0 seconds as "needs improvement," and over 3.0 seconds as "poor." To improve FCP, optimize your critical rendering path, eliminate render-blocking resources, use text compression, and optimize images.
Largest Contentful Paint (LCP)
Largest Contentful Paint measures the render time of the largest content element visible in the viewport. This could be a large image, a video poster, a text block, or a background image loaded via CSS. LCP is arguably the most important Core Web Vital because it indicates when the main content of the page has likely loaded — the point at which users perceive the page as "ready." Google considers an LCP of 2.5 seconds or less as "good," between 2.5 and 4.0 seconds as "needs improvement," and over 4.0 seconds as "poor." Common optimizations include optimizing and preloading the LCP image, using a CDN, implementing server-side rendering, and reducing server response times (TTFB).
Total Blocking Time (TBT)
Total Blocking Time measures the total amount of time between First Contentful Paint and Time to Interactive where the main thread was blocked for long enough to prevent input responsiveness. Long tasks (over 50ms) that block the main thread prevent users from interacting with the page — buttons don't respond, text fields don't accept input, and scrolling may stutter. TBT is a lab metric that correlates strongly with First Input Delay (FID) and the newer Interaction to Next Paint (INP) field metrics. Google considers a TBT of 200ms or less as "good," between 200ms and 600ms as "needs improvement," and over 600ms as "poor." Reducing TBT involves breaking up long tasks, optimizing JavaScript execution, reducing JavaScript payload size, and using web workers for heavy computations.
Cumulative Layout Shift (CLS)
Cumulative Layout Shift measures the sum of all individual layout shift scores for unexpected layout shifts that occur during the entire lifespan of the page. Layout shifts happen when visible elements move from one position to another — think of reading an article and the text suddenly jumps down because an ad or image loaded above it. CLS is critical for user experience because unexpected movement disrupts reading, causes misclicks, and creates a perception of a poorly built website. Google considers a CLS of 0.1 or less as "good," between 0.1 and 0.25 as "needs improvement," and over 0.25 as "poor." Fix CLS issues by setting explicit dimensions on images and videos, reserving space for dynamic content like ads, avoiding inserting content above existing content, and using CSS contain for complex layouts.
Speed Index (SI)
Speed Index measures how quickly content is visually displayed during page load. Unlike FCP which measures a single point in time, Speed Index captures the entire visual progression of the page loading. A page might have a good FCP but then take a long time to fully render all its content, resulting in a poor Speed Index. Google considers a Speed Index of 3.4 seconds or less as "good," between 3.4 and 5.8 seconds as "needs improvement," and over 5.8 seconds as "poor." Improving Speed Index involves the same optimizations as FCP and LCP, plus progressive loading techniques like lazy loading below-the-fold images, using skeleton screens, and implementing progressive image loading.