Read Field Data Before the Lab Score
Distinguish field data from lab data when prioritizing a performance fix.
A green lab score is not the same as a healthy user experience. Field data chooses the priority Core Web Vitals are meant to represent real user outcomes. If p75 mobile INP is poor on a revenue path, that is a stronger priority signal than a perfect desktop Lighthouse run. Segment by page type, device class, and navigation source before naming the problem. Lab data explains the mechanism Once field data tells you where users hurt, use Lighthouse, DevTools, or WebPageTest to reproduce a comparable condition. The trace should answer a narrower question: which resource, task, layout shift, or server…
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