Maybe...Just seemed like he was faster before - the way he looked against Mir and Couture looked a world apart from Cain and Overeem, but they are much better fighters so its hard to judge.
I think it was just who he had. Carwin and Cain rushed him and he went fetal both times. He never really had the chance to look fast, except in the Carwin fight where he basically fell on Carwin and accidently subbed him. With Overeem, he didnt have any range which is understandable against one of the best MMA kickboxers ever. Alister just stood at distance, caught him with kicks, then landed fast knees with great clinch control which hurt Lesnar too quickly for him to potentially catch a leg and take the fight down. THAT would have been a fight though.
But dont get me wrong. I think Lesnar loses every fight yet he has a great record coming out of imo the best line up of fighters of all time.