Having been a post office clerk years ago, I can guarantee you the problem with long lines is 70% the customers. The sheer amount of people who come in, expect a post office member to package their stuff, then argue at every step up is insane. It clogs up the lines quickly. You're also the closest representative to a government employee they know so a lot of that ire gets directed to you. I understood real quickly why post office works develop the attitude they do.
There's also times in the day when people come in mass, usually 3pm onward. You will be waiting longer these periods. I go before or after the lunch hours and it's usually dead.