Which is why it's now called standard mail.
IMO that's not why we changed the names of the classes.
For many many years mail was judged by using 1st Class mail as a standard for delivery speed.
We had 2nd Class which was newspapers and magazines.
There was 3rd Class which was advertising flyers, newsletters, and such which were under 1 lb.
3rd Class could also be sent as a single piece for a rate less than 1st Class.
3rd Class of mail never went by air.
There was also 4th Class for parcel post, book rate, etc.
Changing the name of 3rd Class to Standard Class is a covert attempt to make it's slower delivery expectations the "standard," thus over a few years lowering the publics expectations.
1st Class in now slower than it was 10 years ago.
We created Priority Mail to pretend to give a expectation of quicker deliver.
Actually Priority is a subset of 1st Class.
The "standard" delivery time for Priority anywhere in the US is 2 - 3 days.
Guess what! The standard delivery time for 1st Class anywhere in the nation is 2 - 3 days.
Priority is an excellent gimmick for us, and a rip off for the customer.
(I'm not allowed to say this stuff if you call me on my work phone.)