Well, now you're just not listening. There's no such thing as a single 'traditional Mexican food,' so your anecdotal evidence isn't worth much. Beyond that, though, when I've worked in a Mexican restaurant there were braised dishes that took over a day to complete properly, from scratch. Just like braised dishes from every other cuisine, ever.

There is seriously never going to be a major difference between one country and another in terms of how long it takes to prepare the food, unless you're gauging it based on the wrong food. The question I would start asking is why it takes longer for Central American food than Mexican food, in your experience.