That would drive me crazy, I like things done slowly so I can make sure everything goes smoothly.
A cashier once told me to swipe my credit card while she was still ringing up items. I suppose the computer can handle this, but I found it very strange, as I would have rather waited to see everything ring up, make sure I agree with all the prices (hey, sometimes they come up wrong, we all know that huh!) and THEN swipe my credit card.
I know its important to get people in and out quickly, but when its a matter of maybe a minute, and giving the customer the peace of mind that everything transacted smoothly, I think I would rather wait.