Does speed really matter? Bacteria only eat certain things, right? If some mutated type of bacteria could consume more kinds of matter, wouldn't it be a threat?
Answer: No free lunch, every advantage carries a cost. Any "super" bug we come up with will enter a world with vicious competitors who've been with us since long before the dinosaurs.
Or to put it another way, what happened since bacteria have been around? Humans. Humans started out as single celled creatures like bacteria. And if we can get from that to humans, is it not reasonable to assume that bacteria have had the time to figure out how to multiply as fast as possible?