Google PageSpeed Insights
Google PageSpeed Insights (PSI) runs Google Lighthouse against a URL and adds real-user field data from the Chrome UX Report - showing both what the page scores in a lab test and how it actually performed for real users. It's the tool Google points site owners to when they ask why their site is slow, and it's the closest thing to the official line on Core Web Vitals.
Why It Matters
PSI is the intersection of the lab and the field. The lab score tells you what's broken; the field data tells you what users actually experienced. That combination is exactly what you need to prioritise: a page with poor field LCP but a good lab score points to a hosting or real-device issue, not a front-end one. Without both, you're guessing at which problem matters.
It also matters because it's authoritative. When you're arguing with a client, a developer or a boss about whether the site is fast enough, PSI's numbers are the neutral referee - and they're Google's own numbers, which is hard to argue with.
In Practice
Test the pages that matter - your money pages, not just the homepage. Look at the field data first (real users beat the lab every time), then use the lab diagnostics to find the cause. Re-test after every significant change and keep a baseline so you can prove work moved the number.
Remember the mobile number is the one that counts - that's the experience Google's ranking systems actually measure. And don't chase 100; chase the underlying metrics (LCP, INP, CLS) into the 'good' ranges, which is what the field data actually rewards.
Common Mistakes
Optimising the homepage and calling it done, or chasing a perfect lab score while field data stays red. Test the pages that earn revenue, and fix what real users experienced.
Sources & Further Reading
Where to verify this yourself - Google's own documentation, industry reporting, and how we apply it at Underdog. Don't take our word for it; check the source.
Related Terms
Glossary
Core Web Vitals
Google's page experience metrics - loading speed, visual stability, and interactivity.
Glossary
Google Lighthouse
Google's open-source auditor for page quality - the tool behind the scores people screenshot and the audit trail behind real fixes.
Glossary
Page Speed
How fast your pages load - a confirmed ranking factor that affects conversions too.
Glossary
Technical SEO
Optimising your site's infrastructure so search engines can crawl, render, and index it.
Glossary
User Experience (UX)
How easy and enjoyable your site is to use - bad UX directly hurts rankings.
Know the Words.
Now See Them in Action.
Free teardown. No jargon. Just what's broken and how to fix it.
Get The Teardown