What kind of zombies are you looking for?
Old Style (original)
or
New Resident Evil/ I am Legend Style (Crapy ones)
The old story about zombies are that they are summoned by someone.
Those zombies were slow and didn't have any superhuman strength.
Keep in mind that an original zombie was a human. He is is also probably weak too, by decomposing muscles.
The new ones always appear after some kind of disaster, often medical.
The reason as to why they get super strength, speed and agility beats me.
Perhaps adrenalin or some other sort of disruption in the brain other then the need for blood and flesh.
So if you're working on some kind of medieval sort of game they should be slow but also be able to take a **** load of damage but not do alot of damage.
If it's some kind of new urban mod you're working on you can use them both. Fast or slow doesn't matter.
If they're slow, it could be because some old amulet being found.
And if they're fast, it could be a virus.