No, it has never been answered. There is no setter for moveSpeed on the EntityZombie class nor do the SPEED potion affect them. The tutorial on modifying the behavior of a zombie does NOT address changing the speed.
I would argue it has. Modifying the behavior of a zombie doesn't directly address the issue but provides the tools to do so. This is probably for the better as it forces the user to learn instead of rely on direct answers. /argument If you search for "Modify zombie behavior" or something of the sort there's a thread about how to replace all the NMS zombie's with your own. What you'll need to do is replace LivingEntity (Might be entity) and look for the speed modifier there.
