I did some further research into Roundcube's slow performance when displaying folders with a lot of messages in them, since our
first theory didn't work. I ran some tests while monitoring the IMAP conversations.
I've now found the "culprit" and I'll describe it here.
Here's the issue: If the contents of a folder have changed in any way (even a single message added or removed), then Roundcube doesn't use its existing message index. It fetches the
entire message index all over again, from the server each time. Yikes!
This causes really long delays if you have a folder with lots of messages in it. Every time something changes in that folder, you have to sit there for several seconds while Roundcube re-fetches the whole folder index. Huge drain on the mail server, too.
Why doesn't Roundcube behave like other mail clients in this respect? I know that IMAP must have a way to retrieve only what's changed in a folder (new or removed messages) without having to request the entire list, because I verified that Thunderbird doesn't re-fetch the whole folder.
Has this issue been previously discussed, or submitted for fixing? If not, I would certainly like to put it in the hopper.