I have a 15 rMBP and the shutdown time has noticeably increased. It used to take a couple seconds to shutdown, now it stays in the white screen for up to 30 seconds before shutting down. This doesn't always happen, maybe 50% of the time. Could a program be causing this? I've tried losing everything before shutting down and it still takes a while sometimes. Should I reset PRAM and SMC (whatever that means)?