As for "waiting for people to finish their socialising": I totally know what you mean. I just HATE when people will see their friend in the corridor at school and decide to stop for a chat. These are very narrow, crowded corridors, and it's difficult enough for me to be in crowds (especially crowds interspersed with people who hate me, which makes it like walking through a dodgy neighbourhood during a riot). So I just shove past, and TBH I don't care if it makes them think I'm rude. They should learn some goddamn consideration and continue their conversations somewhere where they aren't inconveniencing other people. I would never stop to talk in the middle of the corridor; me and my friends can walk and talk at the same time if we have to.
Even worse is when the boys decide to have a "play-fight" right in front of me in the corridor, so I'm liable to get hit by accident because they haven't grown up since primary school.
Back to your example...yeah, the cashier was being inconsiderate. It's ok to have a quick few words with each customer, but having a massive conversation when there are people waiting in line is just rude.
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'El reloj, no avanza
y yo quiero ir a verte,
La clase, no acaba
y es como un semestre"