He was his first few years. He was clearly faster than his 40 showed.
In those early years he had everything, elite quickness, vision, balance, and break away speed. But he doesn't have that second gear to pull away from people anymore or outrun the angle. But, he's till an elite back due to his quickness, vision, and balance. And that second gear only matters a few times per year anyway. And, it didn't matter at all as long as they were running him up the middle in the back of our interior OL that wasn't getting any push.
And, you don't need great straight line speed to be very successful in this offense on the NFL in general. That's been proven going back at least as far as Terrell Davis in this offense.