That's several tens of thousands of lines of code that would need changing. A second issue is portability. I'm not sure how portable GTK is or what dependencies it has. Actually, the mouseclick and draw times aren't really that noticeable. I don't have more than a few dozen widgets in any one window, and I put in a window-specific draw function so that if the widget isn't in the active window it isn't redrawn.
GIMP runs great even on a windows box.
All rights reserved.