React Native is a complex web of dependencies and environments that interact in ways often hidden from the developer. We happily build our components and native modules, but rarely see what actually happens under the hood when data crosses between worlds. In this talk, I want to explore the overhead, missed optimizations, and boilerplate we generate every time our app is translated from JavaScript to native. I'll cover the JavaScript Interface (JSI) and Java Native Interface (JNI), and share measurements such as how expensive it really is to pass a single integer from JavaScript into a native component. I'll also look at related types like folly::dynamic and the new (still unstable) jsi::ISerialized fast serialization interface in Hermes. Time permitting, I'll touch on the performance of native modules and discuss when it's actually better to keep something in JavaScript rather than handing it off to native.