Speed is amazing in the NFL. The game is so fast it's mind-boggling. Then time and hits add up, and that guy who was turning on the jets and making everyone else look like they were trying to wade through quick sand slow down, and defenses start hitting them on every play, before they can get to speed, to force their game to change. It does happen to them, and even though they're faster than most, the bridge between the two has dropped considerably.
The reason Megatron was so good in Detroit was speed and size. Strangely enough, as the speed slowed, he became a better receiver. He began to realize he had to have more precise routs, needed to read defenders better, to insure he was on the same page as his QB, and he played with a different type of intensity. That was when he became the best receiver he could be, and quite possibly the best in the NFL, during his period of time in uniform.
I'll take the guy has slowed over the fast guy, if he's learned the game the way it should be played. That said, you need one or two "fleas" who can run that sideline so fast that it scares the bejeebers out of defensive coordinators and the guys who know they have to play him front and back, as well as stop him from coming free if he breaks that route to the middle. These guys can be the scariest, but for the money throw, I go to the Nelson types. They rarely disappoint. I hope, for the team's sake, someone becomes the new Nelson. I'm not convinced they have that guy on the roster, and won't be, until someone steps up and proves they are in consecutive games.