I was looking at the "System Info" applet in Cacko, the third tab labelled "CPU", which is the graph of CPU cycles being used. With nothing going on it sits around 20%. When I start up X/Qt it goes up to around 75% and sits there. Not much left for running apps!
I went and turned all the fast load off on the Cacko apps, but it didn't help that I can see. Might have helped memory usage though.
I didn't find any discussion of this in the forum with a search. I wonder what could be eating all the CPU in X/Qt? Maybe something that can be configured to be a bit more miserly?