Core Web Vitals are Google’s attempt to put numbers on something subjective: how pleasant a site is to use. There are three metrics, they are measured on real user visits, and they contribute to ranking. More immediately than SEO, though, they affect the share of visitors who leave before they ever see your content.
The three metrics in plain terms
The first, Largest Contentful Paint, measures how quickly the main content of the page appears — usually the hero image or the primary heading. Under 2.5 seconds is considered good.
The second, Interaction to Next Paint, measures how quickly the page responds when a user taps something. Under 200 milliseconds is a good value. It replaced the older First Input Delay metric, which only looked at the very first interaction.
The third, Cumulative Layout Shift, measures how much the content jumps around while loading. It is the metric that annoys users most: you tap a button, an ad loads at that moment, the page moves and you tap something else.
Why it matters beyond ranking
The ranking effect is real but smaller than often implied; content relevance remains by far the heaviest factor. The direct effect is elsewhere: in the share of visitors who abandon the page before it finishes loading.
This mostly concerns mobile visits over cellular networks, which is where the majority of traffic sits today. A site that feels flawless at the office on a fast connection can be unusable for half of its actual visitors.
What actually produces results
The interventions with the best effect-to-effort ratio are almost always the same ones, in the order that makes sense to do them:
- Images in a modern format, at the dimensions they are actually displayed, with width and height declared so content does not shift
- Removing JavaScript that is not needed — every third-party library is paid for in response time
- Self-hosted fonts with preloading, instead of an external request that delays text rendering
- Correct caching at the server and CDN for anything that does not change per visit
- Deferred loading of content that sits below the initial screen
- Reserved space for ads and embedded content, so nothing shifts when they arrive
How to measure properly
There are two kinds of measurement, and confusing them leads to wrong conclusions. Lab tools such as Lighthouse run a single test under controlled conditions: useful for comparing before and after a change.
Field data comes from real visitors and is what Google actually uses. You see it in the page experience report in Search Console. When the two disagree, field data has the final say — because it describes the devices and networks of your real users.
A realistic priority
Speed is not an end in itself, and chasing a perfect score rarely pays off. The practical goal is to pass the threshold on all three metrics for real mobile users, and then return to content and to the clarity of your offer — which remain the factors with the largest effect on results.