The person choosing carefully may be planning meals for people you will never see.

Slowness at the front of a queue usually has a reason attached. Somebody comparing prices for a long time may be working out a week of meals on a fixed amount, for a household that is not standing in the shop with them. What looks like indecision is often arithmetic, and the stakes are invisible to everyone waiting behind.

When impatience rises in a queue, let the story of the person ahead be larger than the delay. Give the extra minute without sighing. It costs you very little and is one of the easier kindnesses available in public.