Fucsia's kernel (Zircon) is hard real-time I think, which is what you want if you value predictability and low latency over all else.

Though you don't need an RTOS to make a responsive app, of course. It's just there's such a tendency for systems to bloat and become a tangled mess of RPCs and process activations, while using an RTOS tends to tie your hands and keep you from getting into such a mess.