A different answer to your question is that there should be nothing that causes your computer to slow down just because you have the game installed. It does not load any persistent programs in resident memory and only uses resources when it is actually being run, i.e. when you start the game. There have been memory problems, in Windows to my knowledge, but possibly in the Mac as well, which resulted from the program failing to unload properly but nothing that a system reboot would not fix.