I think the term "speed" gets thrown around a lot, but people often mean different things when they say it.
There's straight line speed, just how fast you can go from A to B.
There's short area quickness, like Welker, Edelman, etc had.
There's mental speed, which can make players LOOK faster because they're playing more decisively. I would put someone like Damien Harris in that category, because he just knew how to recognize a hole and hit it, which made him appear quicker to accelerate than he probably was.
Then there's things like technique and route running which can create bursts of separation, which again creates this appearance of speed. That was the thing (other than health) that I thought Thornton would have to keep working on. We know he had the straight line speed, but if he can't release off the line it doesn't really matter. It's too bad he keeps getting hurt because that's not helping him show improvement there yet.
All this long-winded nonsense to say: There are more ways to get "faster" on offense than just drafting guys with good 40 times. Hell, I would count Bourne as fast, even though no one will mistake him for a Tyreek Hill style burner.