Agree that there are (at least) two types of football fast that are relevant for evaluation of a player like Spiller. Can he flat outrun people? I don't expect him to be caught from behind once he is in the open field.
But secondly, the question is how long does it take him to reach full speed. My understanding is that he is one of these guys, like Gale Sayers, who needs only 1 - 1 1/2 strides to top out. He's gonna be great!