Nico Schlömer wrote:

Aha! This tells me more about what's slow indeed: When clicking on a folder, all the messages are fetched one after the other, the logs spitting out something along the lines of

========================================= *snip*

[...]

[15-Sep-2010 18:21:47 +0200]: S: ) [15-Sep-2010 18:21:47 +0200]: S: * 621 FETCH (INTERNALDATE " 9-Sep-2010 14:19:43 +0200" BODY[HEADER.FIELDS (DATE)] {47} [15-Sep-2010 18:21:47 +0200]: S: Date: Thu, 9 Sep 2010 14:18:59 +0200 (CEST)

[...] ========================================= *snap* =========================================

Each one of those calls takes some time (about 0.07 secs), and for folders with more than 1500 messages in it that's sums up to about 2 minutes. This is were the delay sits. I'm not sure about the implications. Would that mean that the connection to the server is slow?

It looks that your IMAP server doesn't support SORT command, so only solution is to use default sorting. Go to message list options menu and select "None" in "Sorting column" fieldset.