CORE WEB VITALS: WHAT GOOGLE REALLY MEASURES ON YOUR SITE, EXPLAINED WITHOUT JARGON

Since 2021, Google has built three speed measurements into its ranking, the Core Web Vitals. They don't measure an abstract loading time, but what a visitor experiences. Here is what they are, and how to read them.

LCP: the delay before seeing what matters

Largest Contentful Paint is the time that elapses between the click and the display of the largest visible element: the hero photo, the title, the video. It's the moment the visitor feels "the page is there". Good under 2.5 seconds, needs improvement between 2.5 and 4, poor beyond. Usual causes: hero image too heavy, server slow to respond, fonts or scripts blocking rendering.

INP: the reaction delay

Interaction to Next Paint measures the time between a visitor's gesture (a click on a menu, a button, a field) and the page's visible reaction. Good under 200 milliseconds, poor beyond 500. A site can display fast and react badly: that's the case when heavy scripts run in the background, typically page builders, tracking plugins and carousels.

CLS: what moves without being asked

Cumulative Layout Shift measures layout jumps: the button that moves down just as you were about to click, the text that shifts when an image or an ad arrives. Good under 0.1. Causes: images without declared dimensions, fonts loaded late, banners inserted afterwards. It is the easiest signal to fix and the most annoying for your customers.

Where to read them, for free

Two places. PageSpeed Insights, for one page at a time, with lab figures and, if the site has enough traffic, those of real Chrome visitors over 28 days. Search Console, "Core Web Vitals" report, for all your pages rated good, needs improvement and poor: that is the one Google uses. The first reflex, before all that: the free audit tool gives the mobile score and the LCP of your homepage in thirty seconds.

Lab or field?

"Lab" figures come from a simulated phone on 4G: reproducible, useful for comparing before and after. "Field" figures come from real visitors: they are the ones that count for ranking. A site can be good in the lab and poor in the field if your customers have old phones or a bad connection, which is common on mobile on the move.

Does it really change the ranking?

It is not the first criterion: content and links weigh more. But with equal content, the fast page comes first, and above all, the slow page loses visitors before they even read. On mobile, one visitor in two gives up beyond three seconds. The ranking gain is a bonus; the gain in enquiries is immediate.

Where to start

In order: LCP (hero image, server response time, fonts), then CLS (image dimensions, fallback font), then INP (scripts, plugins). The first two can be fixed in a few hours on most sites. If your pages are rated "slow" in Search Console, that's the starting point of the Slow website service.

Frequently asked questions