What are Core Web Vitals (LCP, INP, CLS)?

Core Web Vitals are the three metrics Google uses to measure the real experience of someone visiting a page: how fast the main content appears, how responsive the page is to interaction, and how visually stable the layout stays. Largest Contentful Paint (LCP) measures how long it takes for the largest element visible in the viewport, usually an image or a block of text, to fully render: a good score stays under 2.5 seconds. Interaction to Next Paint (INP) measures the time between a click, tap or keypress and the moment the browser visually updates the screen in response: the threshold is 200 milliseconds, and it is the metric that replaced First Input Delay in March 2024. Cumulative Layout Shift (CLS) sums up how much visible elements move while the page loads, with a threshold of 0.1. Each threshold is meant to hold at the 75th percentile of real page loads, segmented across mobile and desktop.

See also: Performance profiling · Measuring where a running program spends time and memory, to decide what is actually worth optimizing.

Field data versus lab data

The three metrics are measured two different ways, and mixing them up produces numbers that never agree. The Chrome User Experience Report (CrUX) collects anonymized data from real Chrome users and powers PageSpeed Insights and Search Console: it measures all three metrics on real users, INP included. Lighthouse runs in a simulated environment with a single page load and no real user, so it cannot measure INP, since INP requires an actual human interaction; it uses Total Blocking Time as a proxy instead. A green Lighthouse score next to a yellow CrUX score on the same site is not a contradiction: a controlled lab run and the real network, with slow devices and unstable connections, describe different things.

What Google actually says about ranking

Google Search Central documentation confirms that Core Web Vitals are used by ranking systems, but there is no single "page experience signal": it is a combination of minor factors sitting behind content relevance, which remains the dominant criterion. The documentation says it without ambiguity: "Google Search always seeks to show the most relevant content, even if the page experience is sub-par." Core Web Vitals mostly matter as a tiebreaker between pages that are already equally relevant, not as a lever that pushes a weaker page above a stronger one.

A concrete example

An industrial components reseller publishes a category page with twelve uncompressed product photos and a 3 MB banner at the top. On mobile, that page's LCP exceeds 5 seconds, well past the 2.5-second good threshold, and the filter buttons stay inert for a few hundred milliseconds after the first tap, because a third-party script blocks the main thread. The site does not lose measurable ranking positions: the page's content is relevant and the query is not competitive. But it loses visitors before the page finishes loading, and the ones who stay abandon the unresponsive filter, assuming it is broken. The cost is not in ranking, it is in conversion.

Why it matters for decision makers

Anyone weighing a site rebuild should look at field data, not just a lab score produced in a controlled demo: they are different numbers, and only the first one describes what visitors actually experience. Even when ranking does not move, a high LCP or a slow INP stays a direct cost, because a slow site loses customers regardless of where it ranks: nobody waits five seconds for a product catalog when the next competitor loads in two. Measuring where the time actually goes, before deciding what to optimize, is the same discipline covered under performance profiling.