This really isn't true AT ALL, as anyone behind a single cart belonging to an Amishman and his wife buying for the whole community can ruefully verify.
HOWEVER, the reason to avoid express lanes is a dirty little secret well known to all those who've ever worked in a supermarket or high-volume retail outlet: Managers always assign their fastest checkers to the high volume lanes, and their slowest ones to "express" lanes, knowing the slow clerks -- who are usually, paradoxically, the least accurate clerks as well -- will be far less annoying when they only have a few items per customer.
But since the limit for the fast lane at WalMart is twenty items, chances are if you go through the regular checkout, the baskets will be Loooooooaded!