Think of it in layers.
Running the ball forces the safeties up and holds the LBs from chipping the receivers.
Key with the slant forces a triangle of coverage with a corner, OLB and a safety just in case.
Witten will draw OLB and a look from both safeties if he runs a seam or a post.
Jones/Barber with a circle or flat route holds a LB or brings up the corner in zone coverage.
If we run play action with Key running a slant, Witten a seam or post, with the back leaking out, to the side opposite Key, Glenn or Morgan will get single coverage deep, possibly single coverage with a corner releasing them to a safety.
Speed is much less critical than running crisp routes and having the timing of the play unhindered.
If we can dictate the action running the ball and using Key and Witten, we will make plays down field.