Maybe rename it : "World War Y?"
Of all the fast zombies, I like the ones as done in "28 Days/Weeks Later." Weren't they the first "fast ones? Always hoped for a "28 Months" sequel. I want to buy into this movie bad, but they have to help more.
Of all the classic (and cheap crap) slow zombies, I actually think WD does a great job of making them appropriately uber-deadly and a formidable threat while still being so slow.
I even made a geek-out theory as to how there could be fast and slow zombies with varying degrees on a scale, depending on what strain (original) or mutation (rabid) of zombie virus is involved. The slower guys are operating on a viral contamination that is very long-term survival driven above all other functions (and communicates with itself in a more "sentient" fashion, perhaps even saving some of the "buried" intellect of the host--this would play well into some WD story lines, btw) and allows the host ro exist at a very low metabolic rate. It can endure much, including little nourishment for long periods of time.
The more rabid mutation amps up all the motor activity and elevates a berserker state (less communication/sentience), and spurs a more constant and fierce seeking of food (distinguished from the slow, deliberate, inexorable) and feeding behavior (frenetic more than methodical), but maybe will can burn out faster without constant replenishment of sustenance.
Then the diffs get nuanced.