Core Web Vitals, explained for people who sign the invoices
Google grades every page on three numbers. Here's what each one means in plain language, where the pass marks are, and a quick checker for your own site.
If you’ve ever been shown a performance report with a wall of acronyms, this is the short version. Google measures three things about how a page feels to a real visitor, and uses them as a ranking signal. They’re called Core Web Vitals. Each has a pass mark, and the goal is to pass on at least 75% of visits.
The three numbers
Largest Contentful Paint (LCP) is how long it takes for the biggest thing on screen, usually a hero image or headline, to appear. It’s the moment the visitor believes the page has loaded. Pass mark: 2.5 seconds or less. Over 4 seconds is a fail.
Interaction to Next Paint (INP) is how quickly the page responds when someone taps, clicks or types. It replaced an older metric in 2024 because it measures the whole visit, not just the first interaction. Pass mark: 200 milliseconds or less. Over 500 is a fail.
Cumulative Layout Shift (CLS) is how much the page jumps around while loading. That moment when you go to tap a button and an ad pushes it down so you tap something else. Pass mark: a score of 0.1 or less. Over 0.25 is a fail.
Why it matters commercially
Two reasons. Search: pages that pass are eligible for a small ranking boost, and more importantly pages that fail badly are at a disadvantage against competitors that pass. Conversion: these three numbers are a decent proxy for the things that make people abandon a page, and as our speed article covers, the revenue effect of even small improvements is large.
The usual culprits
- LCP: oversized hero images, images without dimensions, fonts that block rendering, slow servers and no caching.
- INP: heavy JavaScript running on page load, chat widgets and tag managers, long tasks blocking the main thread.
- CLS: images and embeds without reserved space, fonts swapping late, banners injected above content.
How to check your own site
Search Console has a Core Web Vitals report for your whole site, based on real visitors. For a single page, PageSpeed Insights shows both field data (real visitors) and lab data (a simulated test). Look at the field data first; the lab number is a diagnostic tool, not a grade.
Every site we build is tested against these thresholds on real devices before launch, and we re-check them after the first month of live traffic. It is far cheaper to keep a fast site fast than to rescue a slow one.
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