Version: 1.9.1 (using KDE KDE 3.5.0)
Installed from: Debian testing/unstable Packages
When KMail is fetching mail, feeding it to it's spamfilter, etc the GUI is not responsive. It is annoying if you want to do a quick round of reading mail if you have to wait 3 seconds each time you flag your mail as spam.
I haven't checked this, but the cause of this is probably because KMail is single-threaded. If these background tasks could be run in a seperate thread it would greatly improve the responsiveness of KMail.
This is probably a duplicate.
Let me introduce you to KDEs most hated bug.
*** This bug has been marked as a duplicate of 41514 ***