Core Web Vitals sound like a diagnostic panel from a hospital drama. They're three measurements of what your site feels like to use, and Google weighs them when deciding where you rank. Here's each one in plain language.
LCP — how fast the main thing appears
Largest Contentful Paint measures how long until the biggest element on screen — usually your hero image or headline — has actually rendered. Under 2.5 seconds is good. Most sites fail this because of one enormous unoptimised hero image, a font that blocks rendering, or a slow server response.
The fixes are boring and effective: serve modern image formats at the size they're displayed, preload the hero, and render the page's HTML on the server instead of assembling it in the browser.
INP — how fast it responds when you tap
Interaction to Next Paint replaced the old first-input metric in 2024. It measures the delay between a user interacting and the screen visibly updating, across the whole visit rather than the first tap. Under 200 milliseconds is good.
Bad INP almost always means too much JavaScript running on the main thread — tag managers, chat widgets, tracking pixels, and heavy client-side frameworks doing work the server could have done.
CLS — how much things jump around
Cumulative Layout Shift measures content moving after it appears. You've felt it: you go to tap a link, an ad loads above it, and you tap something else. A score under 0.1 is good.
It's usually caused by images without width and height attributes, web fonts swapping in at a different size, or banners injected above existing content.
What actually moves the numbers
- Reserve space for every image and embed so nothing reflows when it loads.
- Cut third-party scripts to the ones you genuinely use — each one is main-thread time you don't control.
- Render HTML on the server; ship JavaScript only for the parts that are interactive.
- Self-host fonts, subset them, and set a sensible fallback so text is readable while they load.
- Test on a mid-range Android phone on 4G, not on your laptop on office wifi.
Chrome's Core Web Vitals report shows what real visitors experienced over the last 28 days, which is the number that counts. Lighthouse in your browser is a lab simulation — useful for catching regressions, but it isn't the score Google uses.