2D Rendering On X11 Remains Barely Faster Than CPU Rendering
Martin basically presented on that rendering 2D on a modern X.Org Server is barely faster than CPU rendering, unless compositing. While being barely faster, it consumes more power than CPU-only rendering. But the good news is that more and more software is moving away from X-based rendering.
With the next GTK+ release there will be the GTK Scene Kit, Qt5 already has changed its renderer, and other projects are moving over to purely CPU-based rendering or GPU rendering with projects like Servo's WebRender, Google's Skia, and the new Intel FastUIDraw project.
Toolkits and other projects doing their own rendering have also been moving away from X-based rendering in order to support Wayland where they must be responsible for the CPU/GPU rendering free of the old X acceleration architectures.
Long story short while X.Org rendering isn't very performant, it's becoming less and less of a problem with modern Linux systems. Those wishing to go through Martin's 2D presentation can see these PDF slides or watch the video embedded below (jump to a little past the 5 hour mark).