Page speed, Core Web Vitals and mobile images — explained in plain English
Google doesn't just rank what your site says — it ranks how your site feels to use. That feeling is measured by Core Web Vitals: three numbers that capture loading speed, visual stability and responsiveness. Sites that score well get a ranking edge; sites that score badly bleed visitors before the first headline is read.
The three numbers, translated
- LCP (Largest Contentful Paint): how quickly the biggest visible thing — usually your hero image or headline — appears. Target: under 2.5 seconds.
- CLS (Cumulative Layout Shift): how much the page jumps around while loading. Ever tried to tap a button that moved at the last moment? That's CLS. Target: below 0.1.
- INP (Interaction to Next Paint): how fast the page responds when you tap, click or type. Target: under 200 milliseconds.
Why mobile is where you win or lose
Since 2019, Google uses mobile-first indexing: it evaluates the mobile version of your site to decide rankings, even for desktop searches. And mobile is exactly where heavy sites fall apart — slower processors, weaker connections, smaller data budgets.
Images: the #1 culprit and the easiest fix
On most slow sites, images are the problem. A common mistake: uploading a 4000-pixel photo and letting the browser squeeze it into a 400-pixel slot. The phone downloads ten times the data it needs.
The fix is responsive images — serving each device an appropriately sized file:
- Generate multiple sizes of every image and let the browser pick via
srcset/sizes(modern frameworks do this automatically). - Use modern formats like WebP or AVIF — same quality, a fraction of the weight.
- Lazy-load images below the fold so they don't compete with the content people see first.
- Always set width and height, so the layout doesn't shift when images arrive (that's your CLS score).
Beyond images
- Trim scripts. Every plugin, tracker and widget adds load time. Audit ruthlessly.
- Use a CDN. Serving files from a location near the visitor shaves real time off every request.
- Prefer system-efficient fonts. Two font families, loaded with
font-display: swap, beat five families that block rendering. - Measure with real tools. Google PageSpeed Insights shows your actual field data — the same data Google ranks you on.
The compounding payoff
Speed isn't a one-time optimisation; it's a habit. Sites drift slower as content accumulates — which is why performance checks belong in quarterly maintenance, not just at launch. A fast site ranks better, converts better, and quietly compounds both advantages every quarter it stays fast.