I made a post a while back about getting the Metal backend working on the Mac version of ASH. At the time, on 1.6, the performance was much worse than you'd expect, and so it wasn't really worth it. With the 1.8 boat update, I decided to redo the tests and now it appears Metal is definitely the better backend for Mac.
Results:
| Resolution: 2560x1600 | GPU: Iris Plus 655 | Settings: Perfect | OpenGL (fps) | Metal (fps) |
| On Fishing Ship + No Pixelation + vsync | ~57 | 60 |
| On Fishing Ship + No Pixelation + no vsync | 57 | ~65 |
| On Fishing Ship + Default Pixelation + no vsync | ~230 | ~300 |
| Clump of Trees near Debt Guy + No Pixelation + vsync | ~57 | 60 |
| Clump of Trees near Debt Guy + No Pixelation + no vsync | 57 | ~60 |
| Clump of Trees near Debt Guy + Default Pixelation + no vsync | ~200 | ~250 |
Metal performs up to 30% better. Subjectively, it also has less judder at framerates that are close to, but slightly under 60fps. It does have more judder around 30fps than in OpenGL however.
The one issue with the Metal version is that it sometimes gets stuck in a state where it can't get above 80fps. This can be resolved by going into the settings and toggling between different pixelation options.
One other thing I'd like to make note of is the new color correction option, which appears to slightly desaturate the displayed colors. This leads to different colors in the Metal and OpenGL versions. Since the Metal and OpenGL versions already start with different levels of saturation before any color correction is applied, there are now 4 different levels of saturation available to Mac users. I have no idea which is closest to the artists intent. For more information on how the colors are different in Metal and OpenGL, see my previous post.