Given that there's a very clear trade off in work time between office and street, and that office time, and parts of street time are further categorized as having something to do with administering what's going on, or sorting and handling mail, any given route is going to have different opportunities for productivity improvement.
Turning foot routes into motorized routes enabled every carrier to handle more mail volume, and to get through the route faster. Adding 9 digit zip codes allowed USPS to use machines to sequence the mail for the carrier routes ~ which cut back on office time required to case the mail.
Anyone who's had the slightest experience with carrier service knows these things ~ there are no secrets. The fact you mention none of them suggests you simply don't know enough about the topic to discuss it rationally.
Alas! You sound just like the snotty, arrogant clerks at my post office.
You still did not answer my question.
BTW had two uncles who were postmasters— medium sized towns. You are right about the carrier efficiency being changed with automation, and stopping the relay boxes (which were a security issue too). They carry their entire load with them on the route.
Later the “time and motion” studies came in, the routes got shorter and the staffing issues and on and on— the unions were not of any help. And neither was the internet. Many many people miss the old post office. And many even now conduct their business through it still— for example in town here is a guy who sends out health/vitamin products daily to over 40 or 50 customers a home based small business, bulk mail user. So this still continues. Most people also miss or do not know how important 3rd class mail is as revenue vs. decreasing first class, nor do they know about partnering with FedEx. (not so much UPS which is teamsters throughout).