After hours spent I figured out what is causing the slowdown: apparently, gdk_window_invalidate_region/rect called recursively to child windows (done for the scrolling) is causing redraws to pager and sometimes even the thumb. This is clearly the case of evolution's sidebar.
After hours spent I figured out what is causing the slowdown: apparently, gdk_window_ invalidate_ region/ rect called recursively to child windows (done for the scrolling) is causing redraws to pager and sometimes even the thumb. This is clearly the case of evolution's sidebar.