It's easy to see why you're throwing 91 mph already. This is great:
However, I'd say the whole start of your delivery is a little schizophrenic, but you do get your body into the right position at this point.
I'd recommend this: Keep your initial "up tempo," but step straight backat the start of your delivery instead of all the way toward third base. That's wasted movement, because it takes your whole body off line with the direction of the target, which is straight ahead. Sprinters and pole vaulters, for example, rock forward and backward just prior to leaving the starting blocks -- never side to side. It's the same principle -- you don't want to deviate from the straight line to the target.
Now is stepping straight back going to give you an automatic 2 mph? Probably not. But I think you'll find better consistency and control with your pitches, which will allow you to better utilize that 91 mph fastball and keep hitters off balance better.