EngineeringScoreLighthouse
Core Web Vitals are the measurement our scan does not make. Lighthouse makes it, and PageSpeed Insights is the hosted way to run Lighthouse.
Why our report names it
This is the gap our own speed test page names. We do not measure the Core Web Vitals at all, neither as field data from your visitors nor as a synthetic figure of our own, and the single timing our scan records is the load milestone the browser reports on a cold, unthrottled visit. Lighthouse is the tool for the job we do not do, which is why our remediation copy points at it, and why this page says so plainly rather than hedging about how much of speed we cover.
What it doesn't do
- A score moves while your site stands still. Chrome's own scoring page lists the reasons: adverts or experiments serving differently between runs, traffic routing, the machine you happened to use, extensions injecting scripts, antivirus software. One Lighthouse run is a sample, not a measurement.
- Interaction to Next Paint is not in the laboratory run. Lighthouse weights Total Blocking Time instead, which is a proxy observed during load, so a page can score well and still feel unresponsive to the click a visitor actually cares about.
- Real-user figures are not always there to show. Google says a page needs enough samples to enter the Chrome User Experience Report at all, that PageSpeed Insights falls back to whole-origin numbers below that, and shows nothing at all below that, which is precisely the position a page published last week is in.
- It audits the one address you hand it, in the one configuration you ran it under. Nothing in the report tells you the same template performs differently on the pages you did not audit.
What it does
- Chrome documents exactly what the performance score is made of, which is rarer than it sounds: First Contentful Paint, Speed Index, Largest Contentful Paint, Total Blocking Time and Cumulative Layout Shift, each given a published weight, each converted against a distribution derived from real HTTP Archive data rather than a round number somebody picked.
- The same audits run four ways: the panel inside Chrome DevTools, which reaches pages behind a login; a command-line binary; a Node module you can fail a build with; and a hosted web interface that needs nothing installed.
- PageSpeed Insights is that hosted interface, and it supplies what no laboratory run can. Google describes it as reporting the Lighthouse run alongside the Chrome User Experience Report, so the simulated figures arrive next to what real Chrome users actually experienced for the same address.
More on the problem
Nothing here is gated behind us. Go straight to Lighthouse and make your own mind up.
Sources
Every claim about Lighthouse on this page was read on from the pages below. Products change and this page does not change with them, so treat the date as the claim's expiry rather than its publication.
- Chrome's introduction to Lighthouse: that it is an open-source automated auditing tool covering performance, accessibility and SEO, and the four ways it runs (DevTools panel, command line, Node module, hosted web interface), with the DevTools path covering pages requiring authentication
- The Lighthouse performance scoring page: which metrics carry which weight in the score, that metric scores come from a distribution built on HTTP Archive data, and the listed causes of run-to-run variability
- Google's "About PageSpeed Insights" page: that it reports Lighthouse laboratory data alongside Chrome User Experience Report field data, and that a page with too few samples falls back to origin-level data or to no field data at all
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