When you end up transferring files over a serial port between computers made in 1992 and 2021, I'm not sure if you're crazy or have a strange concept of relaxing.

That said my PowerBook Duo's Printer-modem port seems to support a whopping 57,600 baud. Which works out to about 4.5 ~ 5.2 KB/s using ZMODEM between TeraTerm and SITcomm. Perfect for listening to music or making a sandwich as files transfer.