I wonder how Apple implemented their version though… I do have an observation: if I force VESA graphics on my Hackintosh (this requires special boot flags, so it is easier to do on Hackintosh than a real Mac,) the OS interfaces are rendered very slowly. However if I allow accelerated graphics (default behavior,) it operates smoothly but from time to time the GPU fan would spin up even though there is no GPU-intensive tasks.
I think Apple went down the exclusive OpenGL/Metal route. That is, either get accelerated graphics working at a kernel level, or tolerate slow CoreGraphics.