Lighthouse says your site scores 95, but users complain it's slow. How can both be true?
Understanding the lab/field distinction and why a lab score isn't the user's experience.
Because Lighthouse and real-user monitoring measure fundamentally different things. Lighthouse is LAB data: a single synthetic run on one machine, under a simulated network/CPU throttle, from your location, with a cold cache and no real interactions. It's reproducible and great for debugging and catching regressions in CI — you control every variable — but it's ONE sample of ONE environment. FIELD data (Real User Monitoring, and Google's CrUX dataset) is collected from actual visitors: every device (including cheap phones), every network (including 3G), every geography, every cache state, and real interaction patterns. So a 95 in the lab and unhappy users are perfectly consistent — your lab machine is a fast laptop on fast wifi near the server; your users might be on mid-tier Android over spotty mobile, far from your origin, hitting slow third-party scripts that only fire on real interaction. Key consequences: (1) Core Web Vitals for ranking/assessment use FIELD data at the 75th percentile, not your lab score. (2) Some metrics essentially don't exist in the lab — INP needs real interactions to measure. (3) The right workflow is both: use lab (Lighthouse/DevTools) to find and fix causes reproducibly, and use field (RUM/CrUX) to know what real users actually experience and to prioritize. Trust the field for 'are users happy'; trust the lab for 'what specifically is slow and did my fix work'.
Reconciling a good Lighthouse score with real complaints; deciding which tool answers which question.
LAB (Lighthouse, DevTools) FIELD (RUM, CrUX)
-------------------------- -----------------------------
1 synthetic run millions of real sessions
your machine + simulated throttle every device / network / geo
cold cache, no interactions real caches, real interactions
reproducible -> debug & CI representative -> "are users happy?"
INP not measurable (no input) INP measured end-to-end
p ~ single sample Core Web Vitals judged at p75
Use BOTH: lab to find & verify fixes, field to measure reality.