Hello, I have been using 15" rMBPs for the past four years. Seldom used the 2010 17" (8GB RAM). For the past few days, I started to use it as my main computer. I found it slow. Updating from Snow Leopard to EI Capitan does not help. I read that there is a bug in Mail which slows down the system but I checked activity monitor, it is not on the list of CPU usage. I also don't use that app. Why the machine is slow slow? Is it because I get used to use the rMBPs with SSD?