Grocery store lines aren't usually that much of an issue, mostly because I try my hardest to do grocery shopping when I don't have something pressing to do immediately after - so that whatever lines may be present aren't an annoyance.
What IS an annoyance is when someone at the gas station takes 15 minutes to buy lottery tickets, scratch-offs, etc, and has a full on meeting with the cashier while checking numbers, scratching tickets, and deciding after they win nothing to continue buying more - repeating the process over and over, with zero concept of other people being in line behind them who just need to pay for gas or do something quick. And there's no self-checkout at the gas station.
The universal problem is that people think their time is more precious and abundant than other peoples' time. And don't get me started on the Post Office staying perpetually understaffed, on purpose, at the worst possible times (lunch, and at the end of the day).