I've been experiencing what kmj2318's describing as well. My MacBook Pro (2015) would restart while logging in on a frequent basis. I even started a thread about it here. An Erase & Install fixed it though.That's odd. I don't have that issue on my Yosemite machines. It boots up very quickly and has always been stable for me. There could be something else going on causing your issues. Hopefully, whatever it may be, El Capitan will fix it for you.