performance problems double buffering to 1600x1200 window

[Robert Gibbs <gibbsrc gmail com>]

I am running into a severe performance degradation when using the built in GtkDrawingArea double buffering to a 1600x1200 window.  The program sets a default priority idle handler which calls gtk_widget_queue_draw.  The expose event originally would draw about 1000 polygons, but I commented out the drawing portion and still saw the same poor performance.  The best performance I can get is about 10 frames per second.

If call GTK_WIDGET_UNSET_FLAGS (drawing_area, GTK_DOUBLE_BUFFERED), then use my own pixmap for double buffering, the performance goes up to 50 frames per second.

Can anyone tell me why the built-in double buffering would yield such poor performance?