Firstly you're talking about React Native and at present there is no Android version, so you can only develop iOS applications using React Native. And although the name is shared with React.js and some of the conventions, you actually don't write React.js code to write iOS applications. It's not a write once, deploy everywhere kind of deal.

I have been building an iOS application using React Native and the performance is great (on account it maps to native Objective-C) and it feels like the future. It isn't just like a Phonegap application where it is run within a web container, so you get the native performance. If you already know Objective-C and or Swift, I wouldn't bother with React Native to be honest. RN is good for non-programmers to build iOS and eventually Android apps.