System usage is very low: On my old Odroid C2 it was between <1% and 2% CPU Usage.
I am running all my rules in HABApp and I don’t notice any delay at all from my motion sensor.
It’s possible to measure the round trip time (HABApp → openHAB → HABApp) and it’s typically at around 20-30ms.
If you have troubles or questions you can ask them here or just create a new thread under 3rd party.
It’s the recommended way anyway. If you are running openhabian you can install HABApp through the menu (2B).
It’s a totally different way of creating rules so you have to wrap your head around it in the beginning.
But once you got it it’s easier, faster and less error prone.
If you want to you can also try it on your main machine and connect remotely to your openhab instance.