When you just watch the USPS drivers and the Fedex/UPS drivers you can see part of the problem. UPS/Fedex stop, jump out of the truck, drop off the package, get back in truck, and they are off. USPS stops at my mailbox, he sorts through things for a while, opens mailbox, sorts some more, puts stuff in.. If there's a box that's too large he'll walk over slowly and put it by the door. Go back to the truck.. slowly drive to next mailbox. Maybe stop to talk to the person walking their dog. Rinse. Repeat.
Fedex/UPS drivers have a fire lit under their ass most of the time and have no interest in chatting. Now I understand UPS/Fedex work their guys hard, but if USPS worked with like 1/3rd the urgency they would probably save a lot of money.