You didn't necessarily outspeed Haxorus. Drayden's signature move is Dragon Tail. It hits you, then swaps you out for another Pokemon. Like all swapping moves (Roar, Circle Throw, etcetera), it has reduced Priority. No matter how slow you get, you'll always outspeed a reduced priority move. So, he sent in his ace, and went for a move that slows him down, and got one-shotted, by the sounds of it.

Also, Pluffy is best name.