this seems to affect your
gtk-based applications (pcc, firefox, thunderbird, etc.).
What I did was this (without firefox/thunderbird running):
sluggish logging on/off and starting gtk-based applications? like firefox, pcc, stuff like that?
when my user started getting slow (as i said - gtk-based apps and logging on/off), i deleted all files/folders .gtk*, .gnome* and .gconf*. after that everything is blazing fast again. i'm still not sure why it happens, but kde applications (dolphin, kmail, etc.) never suffered from being sluggish.