How often are USPS on time for deliveries vs Amazons drivers?

As far as I can tell, just as often. No, I don't have the same expectations, but that is more than reasonable considering Amazon has an easier job (every seller already has there product in an Amazon warehouse, as is the USPS has an additional processing step).

That varies substantially by region.

In my area, Amazon's logistics is the least likely to be on time. I've gotten many free months of Prime from delayed packages.

Good point. In my last apartment, Amazon threatened to stop delivering to the entire (large) complex because their delivery people failed so hard at delivering. Packages were left just about everywhere and the Facebook group became more like a lost-and-found package site. Something that didn't happen at all with USPS.

At face value, you argument amounts to "brutally exploiting workers produces better results".

You might be right, but...