My brother have a MPB (mid 2012) and it was running fine with Mavericks. Then he updated to Yosemite. It was very, very slow. Also when I closed the lid on it, without anything running, it wouldn't go to sleep and the fans would run very fast and it would become very hot. I have a MBP (early 2011) and it was a lot quicker (I did a clean install on that one). So we decided to do a clean install on the mid 2012 thinking that something would have gone wrong in the update process. But even after the clean install, without have installed anything on it, it was very, very slow. Got the beachball like every time when doing the most simple tasks. Now we decided to do a clean install once more. I started the installation, and it said it would take like 15 minutes. Now more than 4 hours later, it says it's gonna take 9 minutes more. When doing the clean install on my early 2011, it only took like 30 minutes or so. So I'm thinking it has to be something with the hardware? Did the hardware test on the mid 2012, but it said everything was fine. So don't really know what the problem could be?