Core Web Vitals: What Actually Moves Your Ranking
Most business owners hear "your site is slow" and assume they need a bigger server. Usually that isn't the problem at all. Google grades speed through three specific measurements called Core Web Vitals, and each one has a different cause and a different fix.
Here's what they actually measure.
LCP — Largest Contentful Paint
This is how long it takes for the biggest visible thing on screen to load. Usually that's your hero image or headline.
Target: under 2.5 seconds
The usual culprits:
- Huge uncompressed images
- Slow hosting response times
- Render-blocking CSS and fonts
The single highest-impact fix is almost always image compression. A 4 MB hero photo straight from a camera can often be served at 200 KB in WebP format with no visible difference.
INP — Interaction to Next Paint
This measures responsiveness. Someone taps a button — how long before something visibly happens?
Target: under 200 milliseconds
This one is usually caused by heavy JavaScript blocking the main thread. Common offenders are chat widgets, analytics scripts, and oversized third-party plugins. Audit what you actually need. Most sites load three tracking scripts where one would do.
CLS — Cumulative Layout Shift
This is the annoying one: you go to tap a link and the page jumps because an ad or image loaded late.
Target: under 0.1
The fix is straightforward — always reserve space for content that loads asynchronously:
<!-- Bad: no space reserved, page jumps when it loads -->
<img src="hero.jpg" alt="Hero">
<!-- Good: browser reserves the space immediately -->
<img src="hero.jpg" alt="Hero" width="1200" height="630">
Where to actually start
Run your site through PageSpeed Insights and look at the field data section, not the lab score. Field data comes from real visitors on real devices — that's what Google uses for ranking. A perfect lab score means very little if real users on mid-range phones are having a slow experience.
Then fix in this order:
- Compress and correctly size your images
- Remove third-party scripts you don't genuinely need
- Add width and height attributes to every image
- Only then consider hosting upgrades
In practice, the first two steps solve the majority of problems I see on client sites.