It's not. RN has always used native widgets (with a custom renderer) but the JS code itself is purely interpreted.

They increased the interpreter performance by quite a bit, but JITs are impossible on iOS, so it can never be fast.

We have a RN app that needs to do a lot of geometry processing and things like polyclip are unbearably slow, so we had to add native modules to accelerate them. The web version with a true JIT works just fine.