1.:
None, although I'm not exactly an expert in this area. It saves you harddisk space.
2.:
Aurora only stresses the CPU if you process a turn. The size of the database should be irrelevant for the number of movement and other calculations to process, which are the culprits that slow down a turn. Unless you run multiple instances of aurora at the same time (*and* press the turn buttons simultanously) multiple starts shouldn't slow down processing.
3.:
Gotta admit, the compact database button always crashes aurora for me, on my PC, my laptop and previous OSs I had on my machines.