By drawing I mean the whole frame worth of lines, but only in the one layer. In Animation Paper we call that a “drawing”. The combined “drawings” from ALL your layers we call the “frame”. Drawing is in one frame, but only one layer. Frame is all the layers combined. I hope I’m not messing that explanation up. 🙂 Anyway the lassoed selections are kept in memory for various good reasons as an entity. Having many of them in the same drawing will cause it to slow when you need to pick up or stamp even more.
Undo does the same lasso calculations but in reverse, so it will be slow too in those situations. But again, only when doing these problematic lasso stamps.
So in general, in all other situations, Undo will be fast. Let me know if you find a situation where it is not.