Thanks for the helpful response, I have a shortlist of about 5 things to try now and I'll see how it goes.
First, a few of your questions are easy to answer, given that you're seeing the slowness with many different applications:
Just to clarify, the slowness of GTK in general is one thing. The slowing down of Inkscape is more drastic. I would put it on another level of problem. I cannot continue using it after about an hour and must restart, no other apps behave that way.
Is it more of a GTK thing?
No, not if you also experience the issue with non-Gtk things like Firefox, wxPython, and OOO.
This is quite interesting. I thought Firefox used GTK to draw its gui. It sure don't use QT. wxPython definitely does use GTK and Tomboy uses mono which uses GTK. OOO is anyone's guess :)
The common culprit I would say is GTK. KDE apps give me no trouble at all. They are quick and clean. GTK apps on the other hand ...
Thanks again, Donn.