Performance
Real User Monitoring
RUM
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 consistent?'.
In depth
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.
Why it matters
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.
Also known as
- Real-User Monitoring
- End-User Experience Monitoring
How Lemwatch helps with real user monitoring
Lemwatch monitors real user monitoring continuously and alerts you the moment something changes.
Core Web Vitals monitoring →Related terms
Core Web Vitals
Core Web Vitals are a set of three Google-defined metrics that measure real-user experience on a webpage: Larg…
LCP
Largest Contentful Paint (LCP) measures the time from navigation start until the largest visible element on th…
INP
Interaction to Next Paint (INP) measures the longest single-interaction latency a user experiences on a page, …
CLS
Cumulative Layout Shift (CLS) measures how much visible content unexpectedly moves around the page during load…
Synthetic Monitoring
Synthetic monitoring is the practice of running scripted, scheduled checks against a website from controlled p…
Browse the full Lemwatch monitoring glossary (47 terms).