Maybe kmail does server side filtering and your mailboxes are very large, so that is takes a lot of time and ressources? *t
If it does, it does it automatically. The slowdowns occur on a fresh install without ever having enabled any filtering. This is not a new problem. Every version of kmail I have tried (starting with the one in KDE 3.0) acts the same way. The problem does seem to scale with the size of the directory, but is severe enough that kmail is largely intolerable for IMAP.
Courtney