For years i used Mutt as an IMAP client, but that was honestly quite a nightmare. Even back in the day at uni with an über fast internet connection it was still just not responsive enough. The day i switched to using OfflineIMAP and a local Maildir it was as if the blinds had been drawn aside and sunshine streamed into my INBOX :).

Having said that, i use mu4e these days, because i've become an Emacs user. With a few tweaks it now scratches my itch very effectively (except of course the HTML-email problem as mentioned elsewhere -- this is especially troublesome at work where they want me to include an HTML-embedded-image-tabled monstrosity with about a page of legalese as a signature).

I'd agree with that.

That said there is an obvious solution to slow MBOX folders, which wouldbe to switch to using Maildir-storage, then using the `header_cache` setting(s).