Flutter basically works like a game engine. It has a rendering area and it uses Skia to paint stuff. This means it has a lot of control over what's going on and the bridge to the OS is minimal.
ReactNative is basically giving tons of instructions from the JS engine to the OS. Sure the performance of the OS is great, but this has limitations and many moving pieces.
For example animations used to be a problem in RN because someone thought it was a good idea to send instructions on every frame from the JS engine to the OS. Of course many devices can't keep up with this, and I imagine Facebook solved this by creating an animation engine on the native side of things and JS only passing start/end states and such.
Personally I think Flutter will become the dominant cross platform solution in 2020 or so. ReactNative has always seemed like a hack to me.