Oh, I'm with you mate. I'm far from perfect, but the ignorance and lack of consideration I see on a daily basis still surprises me. I don't get irate, but I've learned to identify a nitwit from a mile away.
Almost opposite of your tale, wherein the cashier was "rushing" you out, I was in a store yesterday, where two "gents" were in line ahead of me. This nitwit (if you can comprehend the intelligence of a "nit", you'll understand my use of the term) waited for all items to be scanned and progress down the conveyor before he ever so casually removed his checkbook, wrote the check (about a minute and a half), and, before tendering the check to the cashier, insisted on first balancing his checkbook (about another minute and a half).
I have several checking accounts, can write a check in about 15 seconds flat, and can balance the register in ten seconds. Why this halfwit failed to consider prewriting the "knowns" (date, signature, payee) prior to the conveyor stopping is the epitome of inconsiderateness (well, not really...I'm just trying to drive the point home). Why this nitwit could not take the receipt and balance the register to the side so that others could continue is the epitome of inefficiency.