My former FIL was a mailman and he used to say the PO demands the carriers work a set amount of hours, regardless of whether they need that much time to cover their route. He used to say he could easily have finished his route in half the time, but they weren’t allowed to go home when they were done. As a customer, I’d rather get my mail in the morning rather than wait til 3 or 4 pm just because the mailman has to stretch it out. Shows how much waste there is at the USPS and how they build in slack time.
Two of the folks I know have said that has changed. The postal inspectors follow them now to determine how long it should take. Both have had their routes extended. My one buddy has to bust his butt almost everyday to get finished in time. Now he does have some slack days but that is only because his mail load is not as high as other times.