Glossary · Performance

Field Data vs Lab Data

Field data is performance measured from real users' visits, while lab data comes from controlled, repeatable synthetic tests; each answers different questions.

Updated October 6, 2026

Field data (also called real user monitoring or RUM) records performance on actual visitors' devices and networks. Lab data comes from a controlled test, such as Lighthouse, run on fixed hardware with simulated throttling.

Why it matters

Each fills a gap in the other. Field data shows what users truly experience across devices, locations and cache states, and it is what Core Web Vitals assessment uses. But it is noisy, arrives after the fact, and cannot explain causes. Lab data is repeatable and detailed, so it suits debugging, pre-release checks and regression tests in CI, but it can miss real conditions such as slow phones, logged-in states and user interactions.

Example

Lab (Lighthouse, one run):   LCP 1.9s
Field (CrUX, 75th pct):      LCP 3.4s

Cause: real visitors include slow Android phones on 3G,
and many arrive via a redirect the lab test skipped.

Metrics only exist in one world sometimes: FID, INP and real navigation timings need real users, while Total Blocking Time and Speed Index are lab-only.

Common mistakes

  • Declaring victory from a single lab score.
  • Using field averages instead of percentiles.
  • Expecting lab and field numbers to match.

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