Hermes is Meta's JS engine optimized for React Native: it precompiles JS to bytecode ahead of time, improving startup time and memory footprint compared to JSC (JavaScriptCore).
// android/app/build.gradle project.ext.react = [ enableHermes: true ]
Hermes has historically lagged slightly behind the latest ECMAScript features and has some `Intl`/`BigInt` support caveats depending on version, which matters for crypto code using `BigInt` for wei-precision math.
Profiling with Hermes' own sampling profiler can reveal hot functions distinct from what a generic JS profiler would show, since bytecode execution differs from JIT-compiled JS.