Not a biological answer: zombie mythology had traditionally presented them as slow and shambling, a deadly threat, but not too difficult to escape if you have an escape route. Think "Night of the Living Dead", right up to "Shaun of the Dead", easily 30-40 years of that portrayal in the zeitgeist. I think it was largely "28 Days Later" that popularised the "fast zombie" threat, which really ups the ante in this genre. "The Last of Us" builds on that strain of the mythology very nicely.

Total fiction, of course, but scary nonetheless.