Core Web Vitals: a guide for people who don't speak numbers

Core Web Vitals are three numbers Google uses to tell whether a page is fast or slow. They are not speed in absolute terms: they are the perception of loading speed, interactivity and visual stability.

LCP: how long until something appears

Largest Contentful Paint measures when the largest visible element appears: often the hero image, often the headline. Above 2.5 seconds the site is considered slow.

The most common cause is not the server: it is an unoptimised hero image. A 2 MB PNG blocks the first render, full stop. The fix is trivial and almost always free: a modern format, declared dimensions, priority loading.

INP: how long people wait after clicking

Interaction to Next Paint replaced First Input Delay in 2024. It measures the response to a real interaction: a click, a tap, a keypress. Above 200 milliseconds the interface feels slow.

Almost always the culprit is JavaScript blocking the main thread. In practice: too much logic in a single file, or a component that re-renders on every keystroke in a form.

CLS: how much the content jumps

Cumulative Layout Shift measures how much the page moves while loading. Above 0.1 the layout is considered unstable.

The typical cause is an image or a banner without declared dimensions: the browser does not know how much space to reserve, so when the content arrives it pushes everything down.

Where to start

Start with data, not with intuition. Install real user monitoring and measure in the field, on your users' devices, not on your laptop over fibre. Then pick a single number to improve and measure it every week.

A website does not become fast through a flash of genius. It becomes fast because every week somebody looks at a number and lowers it.