Google Lighthouse
Google Lighthouse is an open-source tool that audits pages for performance, accessibility, best practices and SEO, scoring each from 0 to 100. It runs in Chrome DevTools, via the CLI, or through PageSpeed Insights. The scores are a snapshot, not a verdict - Lighthouse measures what a page does under test conditions, which makes it a diagnostic starting point rather than the final word on whether a page is fast.
Why It Matters
Lighthouse matters because it turns vague problems into numbers you can track. 'The site feels slow' becomes an LCP of 4.2 seconds. 'Accessibility is rough' becomes a contrast failure with the exact element named. For agencies and in-house teams alike, the scores are the shared language for arguing about page quality - and for proving work actually landed.
The caveat: the scores are a test, not the truth. Real-user metrics (Core Web Vitals in Search Console) are what Google actually weighs. Lighthouse is best used as a fast, repeatable audit - run it, fix what it flags, verify with field data.
In Practice
Run Lighthouse the same way every time - same device profile, same throttling - so the numbers are comparable. Treat a 90+ score as table stakes, then look beyond it: the audits name the specific fixes (render-blocking scripts, image sizes, CLS sources). Fix the named issues, re-run, and watch the score climb as the underlying problems disappear.
Pair it with field data. Lighthouse says a page 'should' be fast; Search Console says what real users experienced. When the two disagree, trust the field data and use Lighthouse to find out why.
Common Mistakes
Tuning the page to pass the test instead of fixing the underlying problem, or screenshotting a perfect score while real-user LCP stays slow. The score is a report card, not the grade that counts.
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
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
Google PageSpeed Insights
Google's free speed tester - the official way to check Core Web Vitals and the first tool Google itself points you to.
Glossary
User Experience (UX)
How easy and enjoyable your site is to use - bad UX directly hurts rankings.
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