Just today my rMBP mid 2013 model is suddenly running as if it only has 500mb of RAM, finders are taking 10 seconds to open dragging windows is jerky and even safari is impossible. I have restarted a few time with no difference in performance. At the moment only activity monitor is open and Safari and the kernel task is showing 600% CPU usage which I don't understand at all. The only thing I can think of is that it is very hot today, currently in the room where the rMBP is operating its 40ºC and the fans are running at about 5000 rpm. However if I check the operating temperatures they are normal, GPU 50ºC CPU 45ºC max temperature of anything is only 52ºC so no problem there. The only thing that is puzzling is this ridiculous kernal_task CPU usage. Any clues?