Hey,

At the moment the calls are blocking on the RN bridge which can be quite slow. It's quite simple to make it both perform well and secure by using a JS based PRNG in addition to the native one.
You can see an example of this approach here: https://github.com/etesync/expo-crypto/blob/master/index.ts

Let me know if you need any help with implementing this or would like a patch.

--
Tom