gnus had some massive IMAP performance improvements a few years (probably close to a decade now) ago. Before that it was quite painful to use it on large mailboxes without a local imap - I used to sync that with offlineimap. When they had a massive issue moving from python2 to python3, and keeping that running on a modern distro started getting painful I tried it without local imap - and realised those improvements made things fast enough that you can run it on remote mailboxes, and even do so in your main emacs instance.