Nice article, but I do not feel that the initial question was really answered.

My theory is: it is part of the obvious increase in speed we saw in action movies over the last decades, which is needed to generate greater thrills. Take explosions/bombs: in the past, the bomb got detonated a few seconds before it exploded; nowadays, the good guys have to basically ride on the shockwave of the explosion. Bullets are not only missed nowadays, but fly by very closely in CGI made slow motion (well, you can make the point that this is speed reduction, but I think this is just in order to dwell on speed). I think we are pretty much at the end of the line here (I guess the only way to top the explosion thing is to show, in a close up, how the firewave burns a few body hairs at the ankle, waiting for that to be shown).

Slow Zombies are just no longer threatening enough, although in the past, Zombies may have been slow in order to dwell a little on their menacity and ugliness ... normal speed would have been boring, and increased speed, well, we know how that looks, from slapstick movies or very old footage (remember the Flying Circus sketch with footage of the queen turning into silent movie slapstick violence, absolutely hilarious).