ATI OpenGL and SetWindowRGN

hi to all.
i developed an application with opengl that works in a window. As control for the scene i use a window with a round shape (obtained with a simple SetWindowRGN call). When i drag the window over the moving scene the application slows down, when i move the windows away the app seems to work fine. I noticed this when i upgraded from an old gts2 to a new radeon 9600…this problem occours with almost ANY windowed opengl app and any non-rectangular shape window…hot to fix? i already have the latest catalyst drivers…

Thank you in advance.