The RB shouldn't be standing with his feet nailed down. When the exchange takes place the RB should be 85% of top speed and then exploding through the hole. He isn't reading linebackers, he already knows where he needs to go. The RB is responsible for the exchange, the QB is in charge of the read. The RB is supposed to go straight to where his gap should be, but he also has the option to cutback to a better gap if it opens up and looks to be the better option. Something that RBs do in any zone blocking run scheme, whether it be under center or out of the shotgun or in the pistol.
The back is most certainly not going 85% of his speed, maybe that is the problem you're finding. Watch the tape and tell me Stew or D'Lo are ever going 85% at the point of hand off.
Also, again, the back's field is narrower because he has to stay closer to the middle of the field in case the QB takes it. So he cant adjust, he cant go faster. . . basically he's wearing lead weights until the QB removes them.
edit: the back doesnt know if he's blocking or running yet at the point of hand off. So, yes, he is reading the play and no he doesnt have a set gap to just run through then. That takes time for a brain to work out and it's about a billion times easier to plant a heel in the dirt and explode when the play is a definite hand off. See a guy breaking through a gap early? cut earlier and bounce to the outside. There's much less time to do that in an option and he has less speed AND is closer to the center of the field.