Motion isn’t immune to limitations of what computers can do. Obviously, the more behaviors you use, the slower it gets.
That’s why there is RAM preview.
I’ve been using Motion for a project recently and it does slow down, especially when you have the timeline open (like on a second monitor). For some scenes I have, it’s real time. For others, it isn’t. For the scenes I have that won’t animate in real time, I end up doing what I do in AE — I render RAM previews.
Like I said, it helps if you don’t have your timeline open. That seems to slow it down when you have projects with a lot of layers, etc.