From Profiler to Production: Measuring What Actually Matters in React & React Native

Every React developer knows how to open DevTools. Far fewer know which numbers to trust — and which ones are lying to you. Lighthouse scores look great in reports but don't always correlate with real user experience. Bundle size matters, but not in the way most teams optimize it. And in React Native, the JS thread can be silently killing your UX while your profiler shows green. I work on the Dynatrace React Native agent — SDK-level instrumentation that runs inside production apps across a wide range of codebases. That gives me an unusual vantage point: not just how one app performs, but what patterns consistently hurt performance across many apps, on both web and mobile. In this talk, I'll walk through the metrics that actually matter — TTI, JS thread jank, startup time, hydration cost — and show how to measure them properly using Hermes profiler, Flashlight, React DevTools, and custom instrumentation. I'll share real before/after benchmarks from web and React Native projects, and explain how to build a measurement culture in your team so performance doesn't regress the moment you stop watching. No generic advice. No "just memoize everything." Real numbers, real tools, real workflows.