Arguably the primary React Native performance issue is that JS is single-threaded. Does React Native provide any useful tools for that?

That's not the primary issue. The main issue that there isn't (or wasn't until recently) a fast, low overhead way to communicate between JavaScript and native code. That's changing with JSI, and I think we might find React Native gets a lot faster in practice if they ever complete the work.

You can run things in workers:

https://github.com/devfd/react-native-workers

To be anal, the root cause of that is using javascript instead of building a native app.

I get that Facebook has the biggest apps on any platform and they run into platform limitations (like number of classes on Android) and build cycles so for them it makes sense to make a dynamic partial-hot-reloading-in-place app development framework, but they're still beholden to the runtime that Apple offers them.

(In theory they could compile JS to a compiled binary that Apple's reviewers are okay with, but I don't know enough about any of that)

The security implications of running JS in an interpreter and running native bytecode are very different, though. Native apps also routinely request much more permissions and gather extra user data such as contacts, just because it is possible to do so.