I don't know, there are so many different ways of holding up a line. Those who wait until their purcases are rung up before they
start to look for their chronically misplaced wallets, for instance, or old grannys and their gazillion cupons, or all the douchebags who can't be bothered to learn their credit card codes, or those who don't have a wallet but store their monies all over their persons...eurgh. Not to mention those who put all their stuff on the belt and then goes off to fetch something they forgot... don't get in line until you're done shopping!!!!1! (And then they return with not one but five items from all over the store.)
Most of my cel phone talking customers are actually able to talk and pay at the same time. I take extra care to establish good eye contact with them though, so they remember where they are and what they are supposed to do.