IMAP is far too chatty, latency kills performance due to numerous round trips. It might be a standard, it might be open, but it's far from optimal. New standards will replace it (hopefully) https://jmap.io/.

FTP has same problem. Telnet was once a standard, time-proven, open protocols - does anyone have preference for that?

You can polish a turd by wrapping it in secure socket (SMTP over TLS etc..), but doesn't address the underlying issues with protocols that were designed for a day long passed.

AFAIK fastmail is driving jmap as a new standard - because they like standards - but like everyone that works with imap implementation for any length of time - don't like imap very much. (That's just an observation based on imap server and client implementations).