Core Web Vitals, explained for people who sell things

Core Web Vitals are three numbers Google uses to describe how a page feels to use. They are worth understanding because they are the rare technical metric that maps closely onto something a shopper actually experiences.

LCP — how long until the page looks ready

Largest Contentful Paint measures when the biggest thing in the viewport finishes rendering. On a product page that is usually the product photo; on a home page, the hero.

It is a proxy for “when could the visitor start using this”. Under 2.5 seconds is good.

What video does to it: a lot, if a video becomes the largest element. That is the failure mode to avoid — not video on the page, but video as the LCP element.

CLS — how much the page moves while loading

Cumulative Layout Shift measures content jumping around as things arrive. A banner that appears and pushes everything down; an image with no dimensions that suddenly takes its space.

Under 0.1 is good. Zero is achievable and worth aiming at.

What video does to it: potentially everything, and it is the most common video-related failure. Cards that arrive without reserved space shove the page down at exactly the moment a shopper is reaching for something. Reserved space fixes it completely.

INP — how quickly the page responds when touched

Interaction to Next Paint measures the delay between a tap and something visibly happening. It replaced First Input Delay because FID only measured the first interaction, which flattered pages that got slow later.

Under 200 milliseconds is good.

What video does to it: less than people fear, provided the player is not doing heavy work on the main thread while the shopper is trying to scroll.

Why they matter commercially, not just technically

They are a ranking signal, which is the reason usually given, and it is the less important one — the effect is real but modest, and it is a tiebreaker rather than a lever.

The bigger reason is that they measure the experience directly. A page with a bad CLS is a page where things move under people’s fingers. A page with a bad LCP is one where people wait. Those cost conversions whether or not anyone is ranking you on them.

What not to do

Do not chase the lab score. A lab test runs on a simulated connection and can be gamed in ways that do nothing for real visitors. The field data — what actual people on actual devices experienced — is the number that matters, and it is the one Google uses.

And do not remove things that earn money to improve a number. A video section that lifts conversion by five per cent is worth a slightly worse LCP. The goal is not a perfect score; it is not paying for features with speed you cannot afford.