Of the bigger name sites, all seem to have their processes in place to get them out ASAP as long as the stuff is in stock. However the bigger problems I find are the postal carriers. Once it is out of the store's hands, it's into the postal system.
I've found that the US seem to be quick so the question, how good is the mexican postal system?
that's weird i noticed that Canada post is good for shipping in Canada, but as soon as it gets into the usps's hands all of a sudden everything comes to a stand still i don't know if it's customs, but it's like shipping into a black hole. all the big shipping companies (dhl, fedex, ups etc.) can get stuff cross border quickly compared to postal so i would recommend talking to whomever your buying from and find out how they are shipping