I've had my 2012 Mac Mini (10.9 Mavericks, Dual Booted W/ Windows) since February 2014 (Late to the party, I know) and, up until about a week ago, it was fine.
Then, all of a sudden, it starts being unusabley slow the moment I try to do anything in finder. Relaunching finder makes it a bit more bareable, and rebooting fixes it completely until I next open finder. I thought this may have been one of those rare times when a Mac gets a virus, however, AVG found nothing. The next thing I found was that it was to do with the fact that my hard drive was full (it was quite full) and that if I was using all of my 4GB of RAM that my Mac would use virtual memory, and that this would effect the speed if I didn't have a lot of HDD space. However, despite removing nearly 1/2 of the things on my HDD, the problem still persists. I have basically ruled out visiting "the local" apple store, as I'm in the middle of nowhere. the nearest one is around 3 hours away.