(.) how long are you working with the editor until it gest slower
(.) Could you send us the .jsp page that you work with inside the editor when it gets slower?
thanks for sending us the layout! We did some testing on our side with your layout and worked 10 minutes with excessive drag and drop in the layout tree. (Un?)Fortunately we could not measure any memory leak (version 20080804, tomorrow's...) - please check the profiler image below. The red line was added by me to demonstrate that the "base memory consumption" is not increasing.
We are very intersted in getting more details on what we do different than you do + contact you tomorrow in parallel to this thread.