Real User Monitoring (RUM) captures actual page load performance in real visitors' browsers — LCP, INP, CLS, TTFB — instead of synthetic tests. RUM answers 'what do users actually experience?' while synthetic monitoring answers 'is the site up and…
Also known as: Real-User Monitoring, End-User Experience Monitoring
RUM works by injecting a small JavaScript beacon that reports Web Vitals back to a collector when the page finishes loading. Because it samples every real session, it reveals problems synthetic checks miss: slow devices, poor networks, geographic tail latency, edge-case browser bugs.
RUM is the source of truth for Core Web Vitals ranking signals — Google uses the CrUX dataset (aggregated Chrome RUM) to decide whether a URL passes the CWV thresholds. Lemwatch pulls CrUX data directly and pairs it with synthetic Lighthouse runs for the full picture.
Without RUM, you optimise for the lab and ship regressions to the field. Slow real-user LCP is what Google actually ranks — synthetic PageSpeed scores are a proxy, not the ground truth.
See it in the product: Core Web Vitals monitoring.