Could also be memory fragmentation, memory paging to disk, or the result of having a fragmented drive.
Defragging the drive could help - the undo system in ArtRage 2 is disk-based and a fragmented drive could slow it down.
Adding more RAM could make a big difference. ArtRage isn't very clever about memory management (but it's on my 'to do' list to improve!). More RAM would be more forgiving.
The Undo buffer is also cleared when you load a project, so you could save, and then reload the same project, without having to exit ArtRage.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"