Can RUM detect downtime?

No. RUM relies on real visitors sending data, so a fully down site produces no measurements and no signal. To catch outages you need synthetic monitoring, which checks the site on a schedule whether or not anyone is visiting.

What is the difference between RUM and synthetic monitoring?

RUM passively measures the experience of real visitors, including field Core Web Vitals across their devices and networks. Synthetic monitoring actively runs scripted checks from a controlled location on a fixed schedule. RUM tells you how the site feels in the wild; synthetic tells you whether it is up and working right now.

Does RUM affect Core Web Vitals or SEO?

RUM does not change your scores; it measures them. The Core Web Vitals it collects are field data, the same kind Google uses as a ranking signal, so RUM shows you the metrics that influence search performance and helps you find the slow segments to fix.

How does RUM collect its data?

A lightweight JavaScript snippet embedded on each page records browser performance timings (using APIs like PerformanceObserver) during real sessions and sends them back for aggregation, typically when the page is being unloaded so it does not slow the visit.

What Is Real User Monitoring (RUM)?

Reviewed by Ionut Caval · Updated June 2026

Real user monitoring (RUM) measures the experience of actual visitors by collecting performance data from their real browsing sessions. It shows how fast a site truly is across the devices, browsers, and network conditions that real people use, rather than from a single test machine.

RUM is the descriptive, real-world layer of monitoring. A small snippet of JavaScript runs in each visitor's browser, records timing data as the page loads and the user interacts, and sends those measurements back for aggregation. Because the data comes from genuine sessions, it captures the long tail of slow experiences that a controlled test rarely reproduces, such as an old phone on a congested mobile network.

What RUM measures

The headline metrics are the field Core Web Vitals, the same user-centric signals search engines use in ranking:

LCP (Largest Contentful Paint): how long until the main content renders; Google's "good" threshold is 2.5 seconds or less.
INP (Interaction to Next Paint): responsiveness to taps and clicks; good is 200 ms or less.
CLS (Cumulative Layout Shift): how much the layout jumps around; good is 0.1 or less.

RUM also captures supporting data like geography, device type, browser, and connection speed, so you can see which segments are slow rather than just an overall average. Reporting on percentiles (the 75th or 95th) matters here, because an average hides the visitors having the worst experience.

RUM vs synthetic monitoring

RUM is the counterpart to synthetic monitoring, which runs scripted checks from a fixed location on a schedule. The key difference is that RUM is passive and depends on traffic: when a site is fully down there are no visitors, so RUM has nothing to report. That is why RUM cannot measure uptime or detect an outage on its own, and why teams run the two together. Synthetic checks answer "is it up and working right now?" while RUM answers "how does it actually feel for the people using it?" For a deeper comparison, see synthetic vs real user monitoring.

In practice, synthetic monitoring (including API monitoring) provides the alerting and availability SLI, while RUM provides the real-world performance picture that guides what to optimize next.

Frequently asked questions

Can RUM detect downtime?

No. RUM relies on real visitors sending data, so a fully down site produces no measurements and no signal. To catch outages you need synthetic monitoring, which checks the site on a schedule whether or not anyone is visiting.
What is the difference between RUM and synthetic monitoring?

RUM passively measures the experience of real visitors, including field Core Web Vitals across their devices and networks. Synthetic monitoring actively runs scripted checks from a controlled location on a fixed schedule. RUM tells you how the site feels in the wild; synthetic tells you whether it is up and working right now.
Does RUM affect Core Web Vitals or SEO?

RUM does not change your scores; it measures them. The Core Web Vitals it collects are field data, the same kind Google uses as a ranking signal, so RUM shows you the metrics that influence search performance and helps you find the slow segments to fix.
How does RUM collect its data?

A lightweight JavaScript snippet embedded on each page records browser performance timings (using APIs like PerformanceObserver) during real sessions and sends them back for aggregation, typically when the page is being unloaded so it does not slow the visit.

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