My first modem was an acoustical one that moved at a blistering 9600 baud.
Blistering indeed. I remember 110bps. Watched my brother download the New Testament - took 9 hours.
That's really fast for an acoustic coupler.
The fastest one I can recall ran at 300 (or maybe 1200).
I remember when I first discovered BBS's in the late 80's. Ran thru several rolls of thermal paper. The paper (and the phone bill) was on the company.