I use POP3, and only wait for mutt when changing to a folder (4s/100k mails). Then everything is basically instantaneous.
But yeah, I don't know about mutt IMAP performance, as I never used it. I've only configured a local IMAP server over my maildir to be able to try aerc. I have no clue why aerc is taking 100-200ms to load a single message (as you see in the video) from the localhost. Maybe naggle (is it even used on localhost)?
Also, I'm not sure how you'd make threads work while loading message list only partially. So full load of the message list is needed anyway (at least once and cache it locally). It's not some failing. I want thread that started 5 months ago to pop forward if new message is posted to it.
EDIT: Faster scrolling loads multiple messages at once faster. Maybe aerc can take into account that user may have scrolled past more than one message in the time the previous message finished loading.