Define scope
Select templates, URLs, devices, regions and important interactions. Record dates, sample volume and intervening changes.
GUIDE / PROTOCOL
Measure the performance people experience
A lab score describes one test; experienced performance depends on users, devices and journeys.
Direct answer
How do you measure performance without mistaking a score for experience?
Separate field from lab data, mobile from desktop, and pages from journeys. Track LCP, INP and CLS distributions, then find the responsible resource or interaction before changing the site.
01 / Review method
Review method
Compare field and lab
Review field Core Web Vitals at the 75th percentile; use lab traces for reproduction and diagnosis, not as a substitute for users.
Find causes
Identify the LCP element, critical resources, blocking scripts, long tasks, layout shifts and third parties. Support each hypothesis with a trace.
Fix and monitor
Prioritize by reach and cost, compare before and after under similar conditions, and watch for regressions.
02 / Evidence to keep
Evidence to keep
Measured population
Source, dates, device, region, URL or page group and sample size.
Distribution
LCP, INP and CLS with percentile, range and field/lab distinction.
Causal trace
Network timeline, affected element, task or resource and tested hypothesis.
Decision
Change, owner, observed effect and monitoring threshold.
AFTER THE REVIEW
Decide from the evidence
Lab results improve while field data stays flat
Do not claim user improvement yet. Check collection delay, population, cache, device mix and URLs before attributing the gap.
Loading is fast but an interaction is slow
Inspect the interactions affecting INP, JavaScript tasks and third parties; retest the same journey after a fix.
03 / FAQ
Frequently asked questions
Why do numbers disagree?
Field data spans real devices and networks; a lab uses a controlled scenario. Compare populations and dates.
What is the 75th percentile?
A value that 75% of measured visits meet or improve on for a given metric and population.
Can one test diagnose INP?
No. Test multiple interactions and use field data; a lab trace explains a particular case.
Should every image be optimized?
Start with images that actually affect perceived loading, including dimensions and priority.
04 / Official references