In this talk, we'll explore how we built comprehensive load testing for React applications at Mattermost, achieving 100,000 concurrent users in production-like environments. We'll begin by revealing why traditional API testing missed critical browser issues that only emerged at scale. Next, we'll demonstrate our open-source tool that uses Playwright to run thousands of real browsers, measuring React-specific metrics like component render times, memory leaks, and state management bottlenecks. Finally, we'll share the optimization journey that reduced browser memory and enabled true production readiness, ensuring our React application performs flawlessly for enterprise customers.