What the three metrics measure
Largest Contentful Paint, or LCP, measures how long it takes for the largest visible element on the page to render. On most pages that is the hero image, the first heading, or a primary block of text. A good LCP is 2.5 seconds or less on a real user device.
Interaction to Next Paint, or INP, measures the latency between a user input and the next visual update. It replaced First Input Delay in 2024 as a more accurate measure of how responsive a page feels. A good INP is 200 milliseconds or less.
Cumulative Layout Shift, or CLS, measures how much visible content unexpectedly moves around during loading. Ads loading late, fonts swapping in, images without dimensions, and injected banners are common causes. A good CLS is 0.1 or less.
Why Core Web Vitals matter
Google uses Core Web Vitals as part of its page experience ranking signal. The effect on rankings is real but modest, usually a tiebreaker between otherwise equal pages rather than a dramatic move. The bigger impact is on conversions. Sites that improve LCP and CLS routinely see double-digit lift in bounce rate, time on page, and revenue per visit.
Web Vitals scores are also reported in Google Search Console, in PageSpeed Insights, and in the Chrome User Experience Report. They are not abstract lab numbers, they are measured from real Chrome users browsing your site.
Common causes of poor Core Web Vitals
Poor LCP is usually a giant unoptimized hero image, a render-blocking script in the head, or a slow first-byte from the server. Poor INP is usually heavy JavaScript on the main thread, often from third-party tag managers or chat widgets. Poor CLS is usually images and embeds rendered without explicit width and height, or banners that push content down once they load.
Fixing Core Web Vitals is rarely a single change. It is a series of small, measurable optimizations, almost all of which also improve the human experience of the site.