Free web performance tools
Two tools, free and without an account. Both report what they measured and what they could not, because a performance number without its source is just a number.
Core Web Vitals checker
What real Chrome visitors actually experienced on your site.
Reads LCP, INP and CLS at the 75th percentile from the Chrome UX Report, on phone, desktop and tablet in one run, and says which window the numbers cover.
- The exact 28-day window the data covers, as dates
- LCP split into its four parts, so you know which one to fix
- Up to 40 weekly periods of history
- Every threshold linked to the Google page it came from
Critical CSS generator
The styles your first paint needs, and nothing else.
Fetches your page and every stylesheet it would apply at the viewport you pick, then keeps only the rules the top of the document depends on.
- Relative url() values rewritten so they still resolve when inlined
- Unused custom properties dropped instead of the whole :root block
- Breakpoints that cannot apply at your viewport left out
- A download that expires on a clock, and a copy that never does
Why these two belong together
The checker tells you whether your Largest Contentful Paint is a problem and which part of it is the slow part. When that slow part turns out to be render delay rather than download time, blocking CSS is usually the cause, and the generator is what you reach for next. A link between the two is why they live on the same site rather than on two separate ones.
Both are the same machinery that produces the page-experience part of our website valuations. Page experience changes how durable a site’s traffic is, and durable traffic is most of what a website is worth.