Indeed, at higher speeds it would waste an awful lot of time waiting for ACKs, and that's why XMODEM ACK Spoofing was a feature of higher end (error-correcting,

Indeed, at higher speeds it would waste an awful lot of time waiting for ACKs, and that's why XMODEM ACK Spoofing was a feature of higher end (error-correcting, obviously) modems. They would recognize the protocol, immediately forge ACKs, deal with error correction and retries, and swallow the other side's ACKs when they eventually came. Truth to be told, by the time this feature was available, many had moved to YMODEM, ZMODEM, BiModem (Bidirectional transfers whoohoo!) and HS/Link (Bidirectional as well, and even chat with the SysOp while transferring data!). But those spoofing modems were very valuable for environments where the protocol was hardwired and you couldn't update.