You do as much of that offline as possible. The "split them into micropolygons" happens at editor time, along with a search tree that makes it really easy to find them.
It's still very demanding! But the goal is that you bake as much of it that you can.
I suppose it depends on what you define "editor time" to be. Importing an asset into a scene live appears to be pretty seamless, at least in comparison to preprocessing stages of yore.
It's still very demanding! But the goal is that you bake as much of it that you can.