I have been following the development of Blender and Metal over the past few months and was excited to see that with Mac OS release of 12.3 that Metal would be supported with older Intel Macs with AMD graphics cards. Before I installed 12.3 on. my 2017 5K iMac I ran some benchmarks on a simple Blender scene and some rather strange results.
On OS 12.2.2
Blender 3.0. 45.55 seconds
Blender 3.1b 49.18 seconds
Blender 3.1 today release 49.88 seconds
Blender 3.1 today release without GPU support 52.23
Blender 3.1 today release with RX580 GPU support 2.15.45
The strange thing is not only is it significantly slower using the Metal but when a start a render I just a spinning lollypop and under force quit it says blender is not responding. If I wait long enough the window will open and the render will complete and reflect to actual total time even including window not open time.
Bottom line here Apple and Blender still have some work to do to implement Metal in Blender.