Sometimes when I press the tray icon wishing to bring Ktorrent to the forefront there will be no response for 5-20 seconds, after which it will come up as if from a deep sleep, with speeds growing back up from 0. Swap thrashing is not an issue, the delay is not related to disk i/o.

KTorrent
Version 4.1.1
Using KDE Development Platform 4.7.1 (4.7.1)

Whether or not the GUI is visible, should not make any difference at all.

Is ktorrent using any CPU when running in the background ?

Yes, normally top attributes 7% CPU to Ktorrent on my 2.20GHz Core2 Duo. What I meant was that it is absolutely unresponsive if I happen to click the tray icon at the wrong time. Top usage remains unchanged as far as I can tell. The windows aren't redrawn until after the fit goes away. It sometimes happens while working in a Ktorrent settings dialog, too (controls are unresponsive), so I cannot relate the hangs completely to just bringing Ktorrent to the foreground.

Is your disk being used a lot during these periods ?

No, not at all. Not any more, than it otherwise would. It's just unresponsive. Other programs like firefox or konsole, can be switched to normally during that time. Just like I wrote in my first posting:

Swap thrashing is not an issue, the delay is not related to disk i/o.

I can't reproduce this, so I have no idea what is going wrong. This is going to be hard to fix without being able to attach a debugger during this period and see what is going on.