When lab says pass and field says fail

When lab says pass and field says fail
Lighthouse (lab) is one cold synthetic run; CrUX (field = real Chrome users) is a p75 over 28 days. Reconcile them methodically.

— Step 1: Pull both — PageSpeed Insights shows CrUX up top, Lighthouse below.
— Step 2: Field worse usually means real users have slower devices/networks than your 4x throttle.
— Step 3: Check device split: mobile p75 often 2-3x desktop. Optimize for the failing segment.
— Step 4: Lab can't see INP from real clicks — never trust lab INP.
— Step 5: Trust field for go/no-go; use lab only to debug causes.

Example: lab LCP 1.8s, field p75 4.2s — the gap was low-end Android.

Takeaway: ship decisions follow field p75, not a single lab number.