I've found RN performance passable but underwhelming. It seems generally worse than a good native UI, and no better than a mobile web UI.
I don't understand why the JS is run in a separate thread, rather than just using the UI thread for UI (as it's intended for). By making all the interaction with the native toolkit asynchronous, it adds extra latency and leads to unavoidable glitches in native widgets used as controlled components.
Animation in React Native is much more of a hassle than UIKit or even CSS. (About the same as native Android, though, where it's also pretty tedious.)
My experience was that RN was way more helpful on Android than on iOS, in general. But that has more to do with Android than RN, which achieves its utility on that platform by papering over a pile of bad decisions, inexplicable awkwardness, and half-finished-apparently-by-the-Summer-interns APIs, none of which should be problems in the first place. I'd seriously consider using it for just for Android even if I wrote a separate, native iOS version of the same app.
The tools are starting to get fairly good on Android. There’s a lot to like about Android Studio. But the actual platform APIs are still a mess.
The extra latency for a layout change is most noticeable if it's an animation hence why you should use native animations not JS polling animations. I agree the Animations API is not as easy as CSS. If you want to do complicated stuff you need a lot of experience with it. This is one of the major hurdles for new RN devs.
Its possible to get good performance our of it, better than mobile web UI, you just need to have some experience with what costs and what doesn't.
I’m working on an alternative to React Native that does exactly this, and it works absolutely fine: React NativeScript.
In NativeScript (and thus also React NativeScript), the JS logic and the UI both execute on the UI thread. Furthermore, you can synchronously access any native API without needing to set up bridging code; bindings to the whole Obj-C/Java runtimes are ready-generated.
If I'm coding in a style that works fine on web, and also works fine in native code, but is slow in React Native, then I'd suggest I'm not the one doing it wrong, it's React Native itself.
This way layout changes are smaller packets so it doesn't saturate the low bandwidth of the thread bridge.
Even if they were to stick with the multithreaded architecture, it should be possible to build a layout structure in the background thread and just hand ownership over to the UI thread. I believe the RN team are working on doing this with their new Hermes runtime (sharing objects directly rather than marshalling them into and out of the JS engine), so that ought to help a bit. But I really think the threading model is just wrong-headed in the first place.
Its possible to get good performance our of it, better than mobile web UI
Is there a case study or even just an example that demonstrates that? I'm skeptical. Mobile browsers are very fast these days, so there's no obvious reason why RN should necessarily be faster.
I suppose RN might have an edge if you're making extensive use of native code. But then calling native code from RN is very laborious, so RN doesn't exactly pull its own weight in that scenario.
I think the reason RN is faster is because mobile browsers have to support decades of crufty CSS in their layout engines while native layout engines are much more lean and ripe for optimisation. React Native uses Yoga layout engine so there may actually be benchmarks available on that.
Totally agreed. I’m working on an alternative to React Native that addresses exactly this: React NativeScript.
In NativeScript (and thus also React NativeScript), the JS logic and the UI both execute on the UI thread. Furthermore, you can synchronously access any native API without needing to set up bridging code; bindings to the whole Obj-C/Java runtimes are ready-generated to begin with.
React native performance is good too it just doesn't come for free. You need to know how to do native animations and you need to do late binding and all the other performance optimisations you should be doing with React anyway.
The only thing that lets React Native down is the tooling. I've had the debugger repeatedly push stale code on refreshes, the source mapping is atrocious and sometimes the network debugger just refuses to work. Also sometimes the app fails to load and you need to reload the app and or restart the debugger and or restart the packager in the right order.