All heavy armor slows you down. Anything but light armor makes you slow, but medium is obviously a happy middle between protection and speed. But if you kill a certain person after a certain quest and get his armor, it's the only power armor in the game that's medium, and if you're going to use the beret, you won't get any - to you special stats. You'll move faster in it than other power armor. But you can only get it if you're in hardcore mode or a mod.