Core Web Vitals: field data vs lab scores (what UK sites should trust)

Lab scores are reproducible; field data is messy but tied to real Chrome users. For ranking and UX arguments with stakeholders, lead with field trends in Search Console — then use lab to debug.

Common false positives

• Perfect Lighthouse on Wi-Fi does not disprove slow 4G field LCP.
• CLS spikes often come from cookie banners, late fonts or embeds — profile with throttling.

Fix LCP element discovery (priority hints, preload where appropriate), reduce CLS from ads and widgets, and keep INP low by deferring non-critical JS.