I like the fast zombie idea.

The whole being of zombies is that they are undead that have risen and reverted back to more animalistic natures.

We are predators, we don't move slow, our food would get away. To me it makes more sense that we'd move fast. We're trying to get our food. Our food moves fast so so do we!

Slow zombies I never got :/ They just let their food get away :/