Well, the overhead on RM is very high. But that is to be expected when you have an app that is trying to do everything.
I would suspect it would get in line before all the other radio messages to be processed. Since the radio is a bottleneck on the system I suspect you'd see that affect this as well.
My point is, that when you take processing the actual automation our of the mix, within the hub is always going to be faster. So, for things where speed is important, its better to make them simple and keep them in the hub. Then, the complex, do this only after checking these 20 other conditions, type of automations, yeah, those would definitely be done somewhere else.
Which makes me just re-iterate an idea I had a long time ago...SmartThings+Hubitat. Call it SmartThings Local Edition. All the speed and local control when you need it (Motion Lighting, HSM/STHM) and the power of the cloud when speed isn't a factor(processing logs, controlling schedules, updating your weather). Add a premium cloud component to Hubitat that would give you access to cloud resources similar to those you get on ST and I think you'd have a game-changer. Now, if I was just a little less lazy...