Your app doesn't need to be single threaded. To begin with, the JS that you write is running on a different thread to the app's UI thread, but if you also have blocking work that you want to offload to another thread, that is possible by writing a native module [1, 2]. The story isn't perfect, there are probably some things that you want to be dealt with asynchronously, but could be written in JavaScript, but on this front React Native isn't any less limiting than native.

[1] https://facebook.github.io/react-native/docs/native-modules-...

[2] https://facebook.github.io/react-native/docs/native-modules-...