The three metrics Google actually measures
Core Web Vitals are three specific measurements Google takes of how your site feels to use, not just how it looks.
- LCP (Largest Contentful Paint) — how long it takes for the main content of the page (usually a hero image or heading) to actually appear.
- CLS (Cumulative Layout Shift) — how much the page jumps around as it loads. Think of a button shifting down just as you're about to tap it, because an ad or image above it loaded late.
- INP (Interaction to Next Paint) — how quickly the page responds when someone clicks or taps something. This replaced an older metric called FID in 2024 and is a better measure of real responsiveness.
Why Google cares about this
These three metrics have been an official ranking signal since 2021, but the more important reason to care about them isn't the algorithm — it's that they directly correlate with whether someone sticks around. A page that takes five seconds to show anything, or that jumps around while loading, gets abandoned. Google is just measuring the same thing your visitors are already feeling.
What actually causes bad scores
In practice, these three almost always come back to the same handful of culprits:
- Bad LCP — usually large, unoptimised images, or a slow server response before the page even starts rendering.
- Bad CLS — images or embeds loading without their final size reserved in advance, or web fonts swapping in and reflowing the text.
- Bad INP — too much JavaScript running on the main thread, often from stacked third-party scripts: chat widgets, analytics pixels, heat-mapping tools, all firing at once.
The simple fixes
None of these require a rebuild to address:
- Compress images and serve them in modern formats (WebP/AVIF instead of raw JPEG/PNG).
- Reserve space for images and embeds before they load, and load fonts in a way that doesn't cause a visible swap.
- Audit every third-party script on the site and cut the ones that aren't earning their keep.
- Avoid heavy page builders that inject their own runtime JavaScript just to render a layout.
How to check your own site
Run your URL through Google's PageSpeed Insights — it's free and gives you the actual LCP, CLS, and INP numbers, plus specific suggestions. If you want the real-world version (not just a lab simulation), the Core Web Vitals report inside Google Search Console shows how your site is actually performing for real visitors over the last 28 days.