Version:           1.11.3 (using 4.2.3 (KDE 4.2.3), Gentoo)
Compiler:          x86_64-pc-linux-gnu-gcc
OS:                Linux (x86_64) release 2.6.29-hh2

I have about 100,000 emails in my account (at least that's what "find .kde/share/apps/kmail/mail says"), and when I check for new mail after kmail started, it becomes so slow that it's almost unresponsive. 

Some time after the mails are downloaded it gets back to normal (nice) speed. 

Subsequent checking for new mail also has a speed penalty, but not as severe as the first check after startup. 

For me this means that I can't just fire up kmail to quickly check my mail. Instead I have to either keep it running at all times or go fetch something to eat/drink after telling it to check my mail (or idling in front of the computer). 

Note: "du -h .kde/share/apps/kmail/mail" reports over 3 GiB.