The almighty drush

So I have been thinking about the theory behind the two drush options pre mill and post mill.

Pre mill drushes hit your enemy earlier and you might be able to catch him off guard, e.g. if he has no loom yet. The community seems to be united in saying the disadvantage is delaying your feudal time.

But why?

In both cases you have the same vil production, same walling effort, same eco cost and even the same cost for barracks and militia.

Am I missing something?