I always dealt with the fast zombies by either nading them or knifing them off a ledge its really the only way to save your own ass from them b hopping bastards >> Sounds impossible but it can be done. The knife thing is a 50/50 chance thos since zombies have a greater knife range >> the slow knife attack has a shorter range that the faster attack so all they do is spam the fast attack for 1 hit kills while the slower knife for humans is the only one that can make em fly so to speak.
bad internet connection may also be a factor, i experience it all the time.