The three metrics, explained plainly
Largest Contentful Paint (LCP) measures how long it takes for the largest visible element — usually a hero image or headline block — to render. Google’s threshold for “good” is under 2.5 seconds.
Interaction to Next Paint (INP) measures how quickly the page responds after a user clicks, taps, or types — essentially, whether the interface feels laggy. It replaced First Input Delay (FID) as the official responsiveness metric, and it’s a stricter, more holistic measurement across the full page lifecycle rather than just the first interaction.
Cumulative Layout Shift (CLS) measures unexpected visual movement — an image loading in and pushing text down after a user has already started reading it, for example. It’s scored on a unitless scale where lower is better.
What typically causes each one to fail
LCP problems usually trace back to unoptimized images, render-blocking JavaScript or CSS, or slow server response times — often compounded on sites still relying on a heavy plugin stack or an unoptimized CMS theme.
INP issues most often come from excessive or poorly optimized JavaScript running on the main thread, third-party scripts (chat widgets, ad tags, analytics tools) competing for processing time, and components that re-render more than necessary in response to a simple interaction.
CLS problems are almost always missing size attributes on images and embeds, web fonts swapping in and shifting text after the page has already rendered, or content — banners, cookie notices — injecting into the layout after initial load without reserved space.
How much this actually moves rankings
Core Web Vitals are one input among many in Google’s ranking systems, and by Google’s own repeated statements, a relatively modest one on their own — they’re more of a tie-breaker between otherwise comparable pages than a dominant ranking factor.
The bigger practical risk isn’t the direct ranking penalty — it’s what a poor score usually indicates: a site architecture that’s also likely struggling with crawl efficiency, mobile usability, and conversion rate, since slow, janky pages perform worse on every one of those dimensions simultaneously, not just search rankings.
Where to actually check your scores
Google Search Console’s Core Web Vitals report shows real-world field data collected from actual visitors (CrUX), which is what Google’s ranking systems use — this is different from a lab test like Lighthouse, which simulates a single load under controlled conditions and can show a materially different result.
Field data is the one that matters for ranking purposes; lab data is more useful for diagnosing and debugging specific issues during development.