one thing I distinctly remember is that when simultaneous two-way Zmodem transfers came out replacing Ymodem, it absolutely blew my mind

(previously all transfers, Xmodem/Ymodem, were one-way with CRC checks on each block slowing things down)

Ymodem-g (I think I remember that correctly) was faster than Zmodem - but if the CRC failed it aborted the whole send. Often I was willing to take the risk. (at 300 baud that adds up)