Version:           1.10.1 (using 4.1.1 (KDE 4.1.1), Gentoo)
Compiler:          x86_64-pc-linux-gnu-gcc
OS:                Linux (x86_64) release 2.6.25-gentoo-r7

kmail responds very slowly to user input - I assume it is caused by many unread mails and my check seems to verify that. 

Background: 

I am used to not only reading mails, but to also collect them from my favorite mailinglists so I can read up / search in it later on. 

Also I tend to leaving spam as unread and doublechecking every unread mesage about once a month before markin it as spam (which I didn't find time for during the last two months). 

That way my unread mail count grew to something above 6000, and kmail grew slow, bordering to almost unresponsive (>10s wait time, until I could see a GUI reaction). 

To test my hypothesis, that the many unread mails are causing that, I marked about 4500 of them as read (1500 unread remain), and kmail became much more responsive (the reaction times for scrolling are below 1/3rd of a second again). 

What happens there in the background which creates that massive amount of load? 

Aside from that, the KDE 4.1 kmail feels much leaner that the 3.5 version (especially because it doesn't block when retrieving mails, I think). Many thanks for your great work! 

Besides: The biggest part of the load seems to have been "waiting" (checked via top).