Zombies typically only move slow when they don't have a target. The slow shuffling of zombies can be regarded as a target acqusition mode. Moving at slow speed allows their less efficient, decaying sensory system more time to interpret sensory input.
Once a target has been identified, however, the zombie can focus all of its attention on pursuit and capture of the target, which is why it doesn't notice other potential targets.
If a group of zombies are biving chase to a group of humans and they manage to grab the rearmost human, the zombies will all attack that human to the exclusion of the others. Once the fallen human has been terminated, the zombies will then attempt to acquire a new target.
This limited interpretation ability also explains why zombies whose eyes are covered act in a very calm, non aggresive fashion. While the eyes are covered, they are unable to locate a target. They may hear potential targets, and will move slowly and turn round and round searching for their prey, but until they are able to see and confirm human movement, they will not attack. Essentially, while their eyes are covered, the world does not exist. Once the hood is lifted, however, its run like a mad zombie time.
Until they catch a skull full of buckshot.
