Understanding Core Web Vitals
Core Web Vitals are a set of metrics that Google uses to measure real-world user experience on your website. Since 2021, they've been a ranking factor in Google Search. The three main Core Web Vitals are Largest Contentful Paint (LCP), which measures loading performance — your main content should load within 2.5 seconds; First Input Delay (FID), now replaced by Interaction to Next Paint (INP), which measures interactivity — pages should respond to user input within 200 milliseconds; and Cumulative Layout Shift (CLS), which measures visual stability — elements shouldn't shift unexpectedly as the page loads, with a target score below 0.1.
Beyond these core metrics, Total Blocking Time (TBT) and Speed Index also impact your performance score. TBT measures how long the main thread is blocked, preventing user interaction. Speed Index measures how quickly content is visually displayed during page load. Our Page Speed Checker uses the Google PageSpeed Insights API to provide the same analysis available in Google's tools, plus actionable optimization opportunities with estimated performance savings.