Performance audit
Satsu measures site speed twice, because the two answers are different questions:
- The performance audit runs Lighthouse against your pages in a controlled lab environment, on a schedule. Same device profile, same network, every run — so two runs are comparable and a fix is provable.
- Web Vitals collects field data from your real visitors, anonymously and cookieless. That's the experience search rankings are judged on.
Rule of thumb: diagnose and verify fixes in the lab; judge reality in the field.
The score
The Lighthouse score condenses the lab metrics into one weighted 0–100 number — the same grade Chrome DevTools and PageSpeed Insights report. 90+ counts as fast, 50–89 needs work, below 50 is slow. A failing Core Web Vital forces the grade down regardless of the score: a site that misses the thresholds real rankings use should not look healthy.
The metrics
The three Core Web Vitals carry the most weight:
- LCP — Largest Contentful Paint: how long until the main content is visible. Good under 2.5 s.
- CLS — Cumulative Layout Shift: how much the layout jumps while loading. Good under 0.1.
- INP — Interaction to Next Paint: how fast the page reacts to input. Good under 200 ms. In the lab, TBT stands in for it, since a lab run has no real interactions.
The remaining lab diagnostics — FCP, Speed Index, TTI, TTFB — locate where the time goes: server, render path, or JavaScript.
Reading the opportunity list
Findings are ranked by estimated savings, worst first. The estimates come from Lighthouse's own model — treat them as direction, not promise. The audit report is Markdown on purpose: copy it into an AI coding assistant and it can act on every finding directly.
Why field numbers differ from lab numbers
Lab runs use a fixed mid-range device on a throttled connection. Your real audience might be faster (desktop-heavy B2B) or slower (mobile-heavy consumer traffic) — so the field p75 can sit well above or below the lab value for the same page. Both are true; they answer different questions.