Why does being in a rush make me rude?

OK, two in a row isn't good for my blood pressure but I have need to RANT again...
Why is it, just because someone doesn't understand that you are in a rush to get somewhere they think you are rude? It can happen anywhere... on the road, walking behind someone along the street, but quite often it happens to me in shops. Particularly supermarkets. You're stood in the checkout lane for baskets only (because you only have a couple of things... you're in a rush, see?) , thinking "Please hurry up, I need to get out of here and back to work / home by x o'clock / back to the garden to bury the body" but low and behold the worlds dummest checkout numpty has just sat down to start her shift. Does she login to her till and start immediately scanning items at breakneck speed, so ensuring swift and efficient service? Does she b*ll*cks... first she farts about getting her seat the right height, then has a quick catch up with Doris who's just vacated said till, asking about her hysterectomy and how her husband George is doing since his little 'embarrassment". Then, just because someone (*cough cough*) makes a comment along the lines of "Oh, dont worry, I'M not in any rush" she looks at you as though you've just pi$$ed on her birthday cake. Finally she's ready to start work and begins the arduous task of picking up items of shopping and holding them above a scanner. I mean COME ON! How hard can that be? Toddlers could do it - Im betting Blind Bill the Man with No Eyes could do it wearing a shagging blindfold. But this blob of makeup and hairspray? Why does it take her THREE GOES to get it right? But the problem is, because of her lack of basic motor skills YOU are the one who will end up in trouble back at work because you weren't back in time, or in trouble at home because you missed dinner or in trouble with the Police because the body has been discovered. So I'M the bad guy for wanting good and efficient service from someone who's only task in her job is to scan my shopping, take my hard earned money, and shut the f*ck up? I tell you all listeners, these are the sorts of people who should be first up against the wall when the revolution comes. I make a plea to you all - do not suffer fools gladly. These people will end up breeding and then where will we be?
Peace out all.