Amazon has NEVER promised/offered me 8-hour shipping.
Amazon is a special case because their tail wags the shipping world.
Out here in the end of the USA, I often get Amazon FREE via UPS in 48 hours *after* Amazon leaks it to UPS. Last time it was 2 days in an Amazon warehouse because 'free', then 2-day weekend, but then BAM 666 miles in 48 hours.
I have not found any to be reliably worst than the others, all through plague and labor shortage. Mild burble when it looked the Teamsters might walk and shippers were fooling with the other guys, but I think the framework is set and strike is unlikely. (If UPS does fall-back on its Managers, just don't buy anything from away.)
As said, a LOT of delay happens at the shipper/store. A lot of vendors will supply false tracking, or pre-print a tracking tag and then never drop the box off at the depot. Amazon routinely lies about this. (It is 'picked up' when it is put by the warehouse door to wait for a truck.)
All, but especially USPS, only update tracking when convenient. My yesterday order was packed in Kansas City (why???) and I don't expect it to scan again until Albany or Boston when that truck is broken-out to local sorting.
At the peak of the pan-panic there were shipping centers, especially USPS, that were just "stuck" for weeks. That seems to be sorted.