mutt itself is kinda slow if you actually use its internal IMAP and SMTP features over the Internet (such as with Gmail). Instead, I used offlineimap to pull down all of my mail to the Mac and pointed mutt at the (Maildir) directory where it was stored (which was incredibly fast, as you might guess).

In addition, instead of having mutt send outbound mail directly to Gmail itself, I fed messages to msmtp (locally) and let that take care of sending them off to Gmail in the background.

That setup isn't for everybody, of course, but I was very happy with it. In particular, {processing/catching up on} all of the mailing lists I subscribe to was much, much quicker.