Core Web Vitals: The Mobile Reality You Never Test

You judge your site's speed on a fast laptop with fast internet. Your visitors don't — most of them arrive on a phone, on a slower connection, and Google ranks the mobile version of your page. My Core Web Vitals checker tests that mobile reality for you: a real PageSpeed run, your three Core Web Vitals with pass-fail colouring, and the real-user field data when Google has it.

What This Tool Actually Checks

Paste a URL. The tool calls Google's PageSpeed Insights API in mobile mode and returns:

• A performance score out of 100, banded — green (90+, well-optimised), amber (50–89, needs work), red (under 50, failing).
• Lab metrics from a single simulated mobile run: LCP (Largest Contentful Paint), CLS (Cumulative Layout Shift), TBT (Total Blocking Time), FCP (First Contentful Paint) and Speed Index — shown as PageSpeed presents them.
• Field data from CrUX (the Chrome User Experience Report) when Google has enough real traffic to your URL: real-visitor LCP, CLS and INP (Interaction to Next Paint), each categorised FAST / AVERAGE / SLOW.

The distinction between those last two is the whole point, and most tools blur it.

Why This Differs From Just "Checking Your Speed"

Two reasons the numbers here mean something a casual speed test doesn't.

First, it tests mobile by default. Phones have slower processors and flakier connections than laptops, so the same page almost always scores lower on mobile — and the mobile score is the one Google ranks on. Test on your desktop and you're measuring an experience few of your visitors have. This measures the one that counts.

Second, it separates lab data from field data, and tells you which you're looking at. Lab data is a single simulated run — fast, repeatable, works on any page including a brand-new one, ideal for testing whether a fix helped. Field data is the truth: real measurements from real Chrome users over the trailing window, and it only appears once Google has enough traffic to your URL. A page can pass in the lab and still fail in the field, because real users are on worse devices and networks than the simulation assumes. A tool that shows you only one number, with no label, is hiding which reality it's reporting. This shows both, clearly marked.

That's the differentiator. The score isn't mine and isn't invented — it's Google's own PageSpeed data, surfaced plainly with the lab/field split that decides whether your fix actually reached real visitors.

What You'll See When You Run It

Enter a page URL and after roughly twenty seconds you get a score gauge — say 42, red — a one-line read on what that means, then three vital cards:

• LCP — 4.1 s, poor (target is 2.5 s or less).
• CLS — 0.02, good (target is 0.1 or less).
• INP — 290 ms, poor (target is 200 ms or less) — shown only if Google has field data for you.

Below that, the full lab metric grid (FCP, Speed Index, TBT) from the single mobile run. The pattern in that example points straight at the fix: a slow LCP and high INP usually mean a heavy hero image and too much blocking JavaScript, not a layout problem (your CLS is fine).

What It Does NOT Do

It measures speed; it doesn't fix it — it won't compress your images or trim your scripts. Field data only appears for pages with enough real Chrome traffic; a low-traffic or brand-new page will show lab data only, and that's expected, not an error. Because it calls the PageSpeed API without a key from a shared server, Google rate-limits it — under load you'll get a "PageSpeed is busy, try again in a minute" message rather than a wrong number, which is the honest behaviour. It also checks one page at a time, and a fast page is necessary but not sufficient: speed rarely outweighs genuinely relevant content. It removes a reason you get held back; it doesn't on its own win the ranking.

Who Should Run This

Anyone whose site feels fine on their own laptop but who has never checked it on the phone experience their customers actually live with. Anyone who's been told their site is "slow" and wants the specific metric to blame. And anyone testing a speed fix — run it before, make the change, run it again on the lab number to confirm the fix landed before you wait on field data to catch up.

Check your Core Web Vitals and see your mobile score with LCP, CLS and INP. To find every speed and structure issue across your whole site, not just one page, the complete website audit covers it in one pass.