I updated from OS X Mavericks to OS X Yosemite on my rMBP (late 2013 model with 16GB ram)and I find that the booting time is very slow compared to Mavericks. When Mavericks was on my rMBP it used to take around 3 seconds to boot now since I have installed Yosemite it boots into a black screen for about 20 seconds before the Apple logo appears.