Interesting, thanks - I remember hearing at one of the WWDC that Coregraphics got a 10x speed improvement using metal. I just read the fine print - it seems draw calls only have a 10x speed improvement, I assume because they render through a layer of some sort. The CA* libraries may use metal - animation and layers, which I guess is where the 10x draw call improvement comes in maybe.
Metal is definitely used extensively in CoreAnimation, and Apple UI tends to rely on that - relatively slow (and memory hungry) rendering of layer content, which is then composited very smoothly and nicely in CA.
They might use it for other stuff like glyph compositing (I think this is one reason they got rid of RGB subpixeling, to make it more amenable to GPU), but last I profiled it, it was still doing a lot of the pixels on CPU, as others have stated.
I believe that the most HW optimizations are made in MacOS on DWM level - window composition and animation.
Window surface is just a bitmap in RAM that needs to be populated by CoreGraphics/CPU.
Some discussion here https://arstechnica.com/civis/viewtopic.php?t=1285571 , though a lot of guessing.