Lighthouse

Lighthouse is a free auditing tool that runs in Chrome DevTools (Audits tab → 'Lighthouse'), as a CLI, as a Node.js library, or as a hosted service via PageSpeed Insights. It generates a report graded 0–100 across five categories: Performance, Accessibility, Best Practices, SEO, and PWA.

The Performance score is the most-tracked one for SEO purposes. It's calculated from a weighted combination of FCP, LCP, Speed Index, Total Blocking Time, and Cumulative Layout Shift. A score of 90+ is considered good; 50–89 needs improvement; under 50 is poor. The score is calibrated to a slow mobile network (4G throttled) and a mid-range mobile CPU, so don't expect your fast desktop tests to match the Lighthouse number.

Lighthouse is a LAB test — it runs your page in a controlled environment, not in the real world. The actual Core Web Vitals Google uses for ranking come from CrUX (Chrome User Experience Report), which is real-user field data aggregated over 28 days. Lighthouse is for debugging; CrUX is for tracking what Google actually sees.

Common Lighthouse failures and their fixes: 'Render-blocking resources' → inline critical CSS or defer non-critical scripts. 'Properly size images' → serve WebP/AVIF at the displayed size, not the full resolution. 'Unused JavaScript' → code-split your bundles, remove unused libraries. 'Avoid enormous network payloads' → cut the total page weight under 1.5MB.

Run Lighthouse on EVERY significant change. Each commit to a marketing page can be tested with `npx lighthouse https://your-page.com --view` to catch regressions before they ship.