Core Web Vitals: the three numbers Google cares about

Site speed isn't just a feeling. Google measures it with three metrics — Core Web Vitals — and uses them as a ranking signal. They also decide how many visitors leave before the page has finished loading.

One thing to get straight before the numbers: Google grades these on real visitors, not on your test run. The bar is the 75th percentile, so three out of four page loads have to land in "good" before you pass. A clean score on your own laptop proves nothing.

LCP — Largest Contentful Paint

When the largest element on screen finishes rendering, usually the hero image or the headline. It's the moment a visitor feels "it's here".

• Good: under 2.5 s
• Needs work: 2.5–4 s
• Poor: over 4 s

Most common culprit: uncompressed images and render-blocking JavaScript.

CLS — Cumulative Layout Shift

How much the content jumps around while the page loads. Nothing is more irritating than clicking a button that moved at the last second.

• Good: under 0.1
• Poor: over 0.25

The fix: reserve the dimensions of images and embeds up front (width/height), and never inject content above what has already rendered.

INP — Interaction to Next Paint

How quickly the site answers a click or a tap. It replaced the older FID in 2024 and measures real responsiveness, not just the first interaction.

• Good: under 200 ms
• Poor: over 500 ms

The main cause of a sluggish response: too much JavaScript running on the main thread.

Where to start if you're failing

You don't fix "Core Web Vitals" in the abstract — you fix the specific thing dragging each number down. In rough order of how often it's the culprit:

• Compress and size your images. An oversized hero image is the number-one cause of a slow LCP. Serve modern formats (WebP or AVIF), and always set width and height so the browser reserves the space — that quietly fixes half your CLS too.
• Cut the JavaScript. Every script the browser parses before it can respond is INP you're spending. Defer what isn't needed for first paint, and delete what you don't use at all.
• Turn on compression and caching at the server or CDN. It's the cheapest speed win there is.

None of these needs a rebuild. They're config and asset hygiene — and they move the exact numbers that move your ranking.

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LCP and CLS can be measured in a lab, and that is where Reconvio gets them — from Lighthouse, translated into concrete findings instead of a table of numbers. INP is the exception. It needs someone to actually click, so no lab tool, Lighthouse included, can hand it to you. That one only comes from real visitors.

Speed is one side of being found; the other is making sure the crawlers — search engines and AI assistants alike — can reach your pages at all. Both are pieces of the same puzzle, which I put together in how to get found in 2026: SEO, AI visibility and site speed.