I have a late 2012 Mac Mini with 16GBs of RAM (of which I use maybe 4 GBs), a 1 TB HD that is not even half full (and which I have verified to be working according to more than one method). The processor uses less than 10 percent of its capacity. I've run First Aid on the disk and tried a bunch of different things to see what the problem is and I cannot for the life of me figure out why it has slown down in the past year. It's even slow using network-less apps (like Excel). I'm running the latest OS X, El Capitan. The only thing I can think of is that perhaps there is an application that is hogging up the HD by writing and deleting or something like that.