Thanks for reporting this. It is not easy to guess what is causing this annoying thing. But it sounds like it is something within the drawing data, since it is stil there after restart and reload of your scene. You mention the lasso. It could be that. Did you get the warning (window coming up, if using the lasso is slow) about that?
We actually just did a “collapse” feature – maybe this will be of great help to you. This will be part of our next release (Alpha 8).
But that doesn’t help you right now. If it is the lasso-problem, then it will definitely help to simply trace your drawing onto a new blank one. You see, the slowness is within a specific drawing. If it is copied (lassoed) to others, then they will be problematic too. So add a new blank drawing, use the light table to trace your drawing onto the new blank. Then delete the old drawing. Then you will see the filesize go way down and the speed be back up to normal again – including Undo. Hopefully this is it and this will help you.
Let me know!