Google PageSpeed Insights (PSI) is a free tool (pagespeed.web.dev) that analyses web page performance and provides scores (0-100) for mobile and desktop across loading speed, accessibility, best practices, and SEO. PSI combines lab data (simulated measurements from a standardised test environment) and field data (real-user measurements from the Chrome User Experience Report). The score itself is directional rather than the primary goal — the actionable diagnostics and specific improvement recommendations below the score are what drive real performance gains.
PageSpeed Insights scores are frequently misunderstood as direct ranking factors. Google's actual performance ranking signal comes from Core Web Vitals field data (real user measurements), not from the PageSpeed lab score. A page with a 62 PageSpeed score but strong field data Core Web Vitals will perform better in rankings than a page with a 95 lab score but poor field data. Focus improvements on the Core Web Vitals metrics (LCP, INP, CLS) rather than chasing an arbitrary lab score number.
Most impactful PageSpeed improvements
- Serve images in next-gen formats — converting JPEG/PNG to WebP or AVIF typically saves 25-80% of image file size
- Properly size images — serving images at the dimensions they are displayed; not larger than needed
- Defer non-critical JavaScript — loading JavaScript that is not needed on initial page load after the critical render path
- Eliminate render-blocking resources — CSS and JavaScript that delay page rendering
- Enable text compression — Brotli or gzip compression of HTML, CSS, and JavaScript files
- Reduce server response time — TTFB (Time to First Byte) should be under 600ms; slow hosting directly delays all other metrics
- Cache static assets — long cache TTLs for CSS, JS, and images reduce repeat load times significantly
Google's PageSpeed Insights categorises scores as Good (90-100), Needs Improvement (50-89), and Poor (0-49). For SEO purposes, the Core Web Vitals thresholds are more important than the overall score — a page passing LCP (under 2.5s), INP (under 200ms), and CLS (under 0.1) with a score of 75 is likely in better shape for search performance than a page with a score of 90 that barely passes Core Web Vitals thresholds. Mobile score is more important than desktop score, as Google uses mobile field data for ranking.
PageSpeed Insights runs separate analyses for mobile and desktop because Google's algorithms simulate mobile and desktop devices differently. Mobile analysis uses a throttled network connection (emulating a mid-range 4G mobile connection) and a slower CPU (emulating a mid-range mobile processor). Desktop analysis uses faster network and processor assumptions. This is why mobile scores are almost always lower than desktop scores — and why focusing performance improvements on mobile first is the right priority.