Let’s think about zombies for a minute. Undead creatures, usually dead humans with an insatiable hunger for human flesh.
I think there are two kinds of zombies; fast ones and slow ones. Note that this post is purely my opinions and is aimed to be more flavourful than precise.
Let’s start with the slow zombies. These are the traditional zombies, raised from the dead by a necromancer. These zombies are reanimated by magic and are forced to do the necromancer’s bidding. I believe they have no objective or motivation on their own, save to do what their master wants so that they can return to their eternal slumber. I think they’re slow because they’ve been rudely awakened and they’re reluctant to do what the necromancer wants. Also, they’ve been decaying for a while, so their motor skills are rusty and their muscles (if any) are weak. So this kind of zombie moves slowly because they lack the motivation and physical capability to move fast.
Now the fast zombies, I feel, are the modern day zombies; living things that have been infected with a highly contagious mind controlling virus. The virus takes complete control of the body and only has one motivation: to spread itself. The virus also has full access to the body’s motor functions and resources and can thus push the body to it’s physical limit and move very very fast.
Both these zombies spread by bite, but while the modern zombie spreads the virus, the traditional zombies spreads the spell or curse that awakened it in the first place, not unlike lycanthropy and vampirism.