perf/field-data-p75

Why

Lighthouse simulates one visit on one device and network; it cannot measure INP and does not see real caches, devices or user behaviour. A green score with failing field data, or the opposite, is common. Saying "the page passes Core Web Vitals" after a lab run misleads the team and the client of the site.

Fix

Report lab results as "Lighthouse (lab, mobile preset)" with the version and the median of several runs, and use them to find causes. Report vitals from the field: PageSpeed Insights' field section or CrUX for public pages with enough traffic, or your own web-vitals data, at the 75th percentile, mobile and desktop apart.

Examples

bad:  "Performance fixed: Lighthouse 98, Core Web Vitals passed."
good: "Lab (Lighthouse 13.6, mobile, median of 3): LCP 2.1 s, TBT 120 ms. Field (CrUX, 28 days, p75): LCP 2.4 s, INP 180 ms, CLS 0.04."

Sources

• web.dev, Web Vitals (field and lab, 75th percentile)
• web.dev, Why lab and field data can be different

Read it in the terminal

npx github:dawidolko/frontend-skills explain perf/field-data-p75