Real User Monitoring (RUM) is the practice of collecting performance data from actual visitors as they interact with a site, rather than from synthetic tests run in a controlled environment. A small JavaScript snippet records metrics like [First Contentful Paint (FCP)](/glossary/first-contentful-paint), load times, and interactivity across every real session and device. RUM reveals performance problems that only appear under specific real-world conditions — slow mobile networks, older hardware, geographic distance from servers — that lab tools like Lighthouse may miss. Many analytics and observability platforms offer built-in RUM capabilities.