improving Mail.app performance on macOS Catalina

Hi, folks,

I have been struggling for the last few months with the issue of macOS Mail.app performance, particularly the issue where it opens up many, many IMAP connections and starts slowing to a crawl.

In my case, the issue appears to have been solved by ensuring that the message count in my INBOX, Sent and Archive mailboxes--the ones mentioned in Mail.app's configuration--is kept reasonably low, below a few thousand messages. I had tens of thousands of messages in all of them. That, mixed, with rules that would label and move messages around, appears to be the culprit in the number of IMAP processes that are spawned.

I didn't delete the mail. I have a server-side shell script which automatically moves mail from my "active" Archive and Sent mailboxes into long-term ones that are not involved in mail delivery at all. That seems to have done the trick.

I also configured my dovecot IMAP mail servers (which I run myself) to limit the number of processes per user.

Happy to post those resources here if anyone finds them useful.