why does thread grouping take so much cpu time?

[Dave Stevens]

Kmail 1.13.5
using KDE development platform 4.4.5 (KDE 4.4.5)
and Fedora 13_64

and dual core Opteron 4G ram lotsa free disk.

Whenever I try to read a folder with a lot of entries (20k or so) cpu usage 
spikes for about two or three minutes and the only activity I can see is 
thread grouping. I don't understand why this takes so long. Is there something 
I can do about this? It makes the machine only slightly usable until it's over 
or until I give up in disgust and click on some other much smaller folder just 
to get my computer back. Very frustrating.

Dave