Site speed: Core Web Vitals without the jargon
The three measures Google uses to judge how pleasant your site is to use, what they mean in practice, and the most common reasons they're bad.
A slow site doesn't look like a problem until you open it on a phone, on mobile data, somewhere outside town. That's when you see what the customer sees: a blank screen, text that jumps around, a button that doesn't respond.
Google measures this through three indicators it calls Core Web Vitals.
Three measures, in plain language
| Measure | What it measures | Good is |
|---|---|---|
| LCP (Largest Contentful Paint) | How quickly the main content appears, usually a large image or heading | up to 2.5 s |
| INP (Interaction to Next Paint) | How quickly the page responds when you click or tap something | up to 200 ms |
| CLS (Cumulative Layout Shift) | How much the content "jumps" while the page loads | up to 0.1 |
Two things worth knowing:
- Google grades real visits from Chrome, not a lab test. For a page to pass, at least 75% of visits have to be in the good range.
- INP replaced the older FID metric in March 2024. If someone is still talking to you about FID, their information is out of date.
How much does it matter to Google?
Core Web Vitals are part of what Google calls "page experience" and do feed into ranking, but relevant, useful content matters more. A fast site without good content won't outrank excellent content on a slower site.
Where speed really decides things is what happens after the click. Someone staring at a blank screen on their phone doesn't wait long.
How to check your site
- Open PageSpeed Insights and enter your site's address.
- Look at the top of the report first, with real user data. The lower part is a lab test, useful for diagnosis.
- If your site has few visitors, there may be no real-user data. Then you rely on the lab test, bearing in mind it's only a guide.
- In Google Search Console, the "Core Web Vitals" report shows which groups of pages have a problem.
The usual suspects
Oversized images
A 5 MB photo straight off a phone, shown in a 400-pixel square. The most common problem and the easiest to fix: WebP or AVIF, images at the size they're displayed, and lazy loading (loading="lazy") for everything below the first screen.
Too many plugins and heavy page builders
Every plugin, slider, chat widget and pop-up adds code the phone has to download and run. That usually hurts INP the most.
Third-party scripts
Analytics, ad pixels, embedded videos and maps. Each one loads its own code from someone else's server. Keep only what you actually use, and load maps and videos on click.
Content that jumps
Images without set dimensions, banners injected at the top after loading, fonts that change the size of the text. The fix is to reserve space for each element in advance.
Slow hosting without a CDN
If the server is slow or far away, everything else is late. A static site on a CDN is served from a location close to the visitor.
How I do it
On this site, for example, I built the globe on the home page from scratch instead of loading a ready-made 3D library. The version with the library weighed about 240 KB; this one is about 6 KB. Decisions like that are invisible, but you feel them on every phone.
If PageSpeed shows you red numbers and you don't know what to do with them, get in touch for a free twenty-minute call and we'll go through them.