As is, the fast zombie is useless in most maps anyway because being fast means nothing when trying to squeeze through a narrow corridor to get at camping humans. The low HP of this zombie class means you can sometimes shoot it down without reloading, not even needing unlimited ammo.
Sadly, even when chasing a human out in the open, the fast zombie isn't all that fast. I remember running behind a human who didn't seem to realize I was there for a good 30 seconds in Osprey Escape before without ever catching up to him. Also, when running along other zombies it's not as though the fast zombie cruises by them with its speed, usually it ends up barely ahead of any other zombies running along with it.
My opinion is that if this zombie is chasing a human out in the open and doesn't get shot to slow it down, it should gain extremely quickly on whoever it is chasing. Otherwise, why have this zombie class at all?