In this, the digital age, everything is being steadily optimized for maximum efficiency. There was a time when cashiers laboriously entered the price of each item into their registers and tallied up the total. They then waited patiently while the customer wrote the amount on a check in longhand (maybe even in that ancient script known as cursive) and recorded the transaction in her log.
Nowadays, lasers read the bar codes of product after product, barely giving you time to brace yourself for the shocking total before you slip a little piece of plastic into a waiting slot, key in a code and pretend it's all free.
But some people are old school. Some people still use checks. More often than not, these luddites are old hands at the practice and they have everything all filled out except for the total (or at least they should). That's just good manners, and in such cases the transaction will take no longer than would a credit card payment.
But if, for some reason, the process drags on and on and you're standing there waiting and feeling like your feet are going to fall off, just remember that while the person in front of you hasn't figured out debit cards yet, at least he's not living on credit. And that deserves the same respect and patience as any other customer (unless he's planning to bounce that check).