PageSpeed Insights (PSI)

Definition

PageSpeed Insights (PSI) is Google's free, public web performance analysis tool available at `pagespeed.web.dev`. It analyses a URL and returns two types of data: (1) **Field data** — real Chrome User Experience (CrUX) data from actual users who visited that URL, showing 28-day aggregates for LCP, INP, CLS, FCP, TTFB, and FID (legacy). This field data is what Google uses as Page Experience ranking signals — it's labelled "Discover what your real users are experiencing." (2) **Lab data** — a Lighthouse test run in a controlled, simulated environment (throttled 4G connection, mid-tier mobile device emulation), producing scores and metrics including Performance score, FCP, LCP, TBT, CLS, and Speed Index. PSI is distinct from Lighthouse (which only provides lab data) and from GSC Core Web Vitals report (which provides field data but only for pages with sufficient CrUX traffic). PSI is the only place that shows both together for a single URL, making it the most useful standalone diagnostic for understanding both the ranking signal (field data) and the specific bottlenecks to fix (lab data with opportunities and diagnostics).

Why it matters for SEO

PSI is the go-to free tool for diagnosing Core Web Vitals issues because it shows the gap between what real users experience (field data) and what a controlled test measures (lab data). A common confusion: a page can have a 95 Lighthouse Performance score but show "Poor" LCP in field data — because Lighthouse doesn't replicate third-party scripts, real user device diversity, or CDN caching behaviour. The field data is what matters for ranking; the lab data points to what to fix. Interpreting PSI: start with field data (is LCP/INP/CLS Good?); if Poor or Needs Improvement, look at the lab data Opportunities section for specific fixes (eliminate render-blocking resources, preload LCP image, reduce unused JavaScript); implement fixes and monitor field data improvement in GSC over the following 28-day window.

How DeepSEOAnalysis checks this

The audit integrates with the PageSpeed Insights API (`PSI_API_KEY` in environment config) to fetch both lab and field data for crawled pages. PSI field data (LCP, INP, CLS percentile values and status) is displayed in the performance section of the report alongside the Lighthouse scores from the audit's own crawl-time Lighthouse run. Pages without sufficient CrUX data (low-traffic pages) show field data as "Not enough data" — the lab metrics are still reported. The PSI integration covers up to the crawl's page limit; high-traffic sites should also check the GSC Core Web Vitals report for aggregate field data across all pages.

Useful tools and resources

• Full performance audit