Core Web Vitals, without the jargon

Core Web Vitals are three measurements Google uses to judge how a page feels to the people using it. They sound technical, but each one answers a plain question a visitor would ask.

01 · LCP: does the main thing show up quickly?

Largest Contentful Paint is the time until the biggest piece of content on the first screen appears: usually the headline or the main image. Google’s “good” threshold is 2.5 seconds or less (web.dev).

Usual causes of a slow LCP: a huge hero image, too many fonts, a page builder loading lots of scripts before anything shows, or slow hosting.

Usual fixes: serve images in modern formats (AVIF or WebP) at the size the screen needs, preload the one or two fonts the first screen uses, and make sure nothing blocks the page from painting.

02 · INP: does the page respond when I tap?

Interaction to Next Paint measures how long the page takes to react when someone taps a button, opens a menu or types. “Good” is 200 milliseconds or less. INP replaced the older First Input Delay metric in March 2024.

Usual causes: too much JavaScript, often from chat widgets, tracking tags and sliders, all competing for the phone’s processor.

Usual fixes: remove scripts you don’t use, load the rest after the page is interactive, and be wary of anything that runs on every scroll.

03 · CLS: does the page stay still?

Cumulative Layout Shift measures how much things jump around while the page loads: the button that moves just as you tap it. “Good” is 0.1 or less.

Usual causes: images without dimensions, fonts that swap and reflow text, and banners or pop-ups injected at the top after the page loads.

Usual fixes: give every image a width and height, reserve space for anything that appears later, and keep late-loading content below what’s already on screen.

How Google judges them

Google doesn’t use your fast office Wi-Fi. It uses data from real Chrome visits, and a page passes when 75% of visits meet the “good” threshold, assessed separately for mobile and desktop. A site that’s fine on your laptop can fail on a mid-range phone on mobile data, and that’s the one that counts.

How to check your own site

1. Open PageSpeed Insights and enter your page. The top section shows real-user data if your site has enough traffic; the lower section is a lab test on a simulated slow phone.
2. In Google Search Console, the Core Web Vitals report groups your pages into good, needs improvement and poor.

Low-traffic sites often have no real-user data yet. Then the lab test is your best guide, and its mobile score is the one to watch.

Why it matters for ads, too

Speed isn’t only about search ranking. If you pay for every click, a page that takes five seconds to show anything loses some of those clicks before they see your offer. That’s money spent on people who never saw the page.

This site is my own test case: it loads its headline in about 2.3 seconds on Lighthouse’s simulated slow phone. Here’s how it was built. If your site is slow, website development starts with exactly these three numbers.

Three numbers: does the main content show fast, does the page respond when tapped, does it stay still. Google judges them on real visits, at the 75th percentile.