I actually find the slower zombie to have a much more effect than the fast running ones. When I see a zombie running it looks to be a real person, the only "good" running zombie I saw was the priest in 28 days later, the rest are obviously extras hired to sprint in front of a camera.
With a walking zombie it's very easy to believe that these things are dead, and when they are in big numbers (Which Romero shows very often) they are very dangerous as seen in the numerous feasting scenes....something that was missing from the remake.
Now how this spread so fast was easily shown in the original Dawn of Dead, with the apartment complex. The people were very attached to their relatives who recently became zombies that they couldn't kill them and inevitably were bitten them selves. This vicious cycle lead to the epidemic.