If you collapse something to function and then Undo, it takes an extremely long time to execute. I tried to undo a collapse of 7 nodes and it took at least 5 minutes to undo. It had one execution input and one Int variable left on the outside that was fed to 3 items on the inside. I hit undo because I only wanted 1 int input instead of the 3 on the outside.
I have seen this a few times before. I want to say every time I have felt the need to undo a collapse, which is rare enough.