Glossary · Monitoring

RUM (Real User Monitoring)

RUM collects performance and error data from real visitors' browsers as they use a site, showing the speed and failures users actually experience.

Updated October 6, 2026

Real User Monitoring (RUM) measures how a site performs for actual visitors. A small script in the page reads browser timing APIs and sends metrics such as load time, Core Web Vitals and JavaScript errors back to a collector, tagged with device, browser and country.

Why it matters

Synthetic checks run from fixed locations on clean machines, so they miss the variety of real devices, networks and extensions. RUM shows the spread: a page might be fast on desktop fibre and slow on mid-range phones in another region. It also reveals problems that only appear with real traffic and real user behavior. The two approaches complement each other, with synthetic monitoring giving a consistent baseline and alerts when no one is visiting.

Example

A RUM dashboard shows:

LCP p75: desktop 1.9 s, mobile 3.8 s
JS errors: 0.7% of sessions, mostly Safari 15

The mobile gap guides where to optimize, and the Safari errors point at a compatibility bug.

Common mistakes

  • Relying on RUM alone. With no visitors at 3 a.m., it cannot tell you the site is down.
  • Collecting personal data without consent.
  • Reading averages instead of percentiles such as p75 or p95.

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Keep reading

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