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Also, KDE/Qt apps can be used with the other Xorg server: XFBdev with little performances penality while Gnome/GTK app can't be used at all with resolution higher than 2x2. Qt is able to ask the framebuffer to update a bunch of pixel (at once) directly, while GTK redraw the whole screen pixel per pixel. So I can browser the web or watch a video on the framebuffer or scroll thumbnails in dolphin at more than 24fps while GTK can not even draw them in less than 2 second per frame.
It is when you really see how well Qt work and how well it is integrated in X.
You have no GPU but a lot of CPU? Fine, it will use the raster backend and draw the window as if it was a moving image (bitmap).
You have normal needs or networked ones? Fine, the X11 backend will use the protocol in a efficent way to draw the window (like in my xfbdev example)
Have a good GPU or want to save energy in a smartphone with efficient GPU and hungry CPU? The hardware accelerated OpenGL backend is there for you (but a little experimental).