Since they use their own Direct3D on top of Metal (D3DMetal), you can immediately start investigating performance while not much of the game is ported yet. Even while running the game in Wine, you can already analyze graphics performance with Instruments.app.
At least this was the messaging in their talk about this: first you had to work months on a port to see how well a game would work on Mac hardware, now you can immediately run it, investigate bottlenecks and see where a port would land performance-wise before putting in any of the work.
I wonder if they will make the D3DMetal usable from native code as well, I can imagine that would make porting much easier:
- First you run on Wine + D3DMetal to get idea if your game would run well enough on Apple hardware at all.
- Then you start building the game natively, still using D3DMetal, so that you don't have to immediately port the graphics stuff.
- Once you have a running game on top of D3DMetal, you start porting it over to Metal with their shader converters, etc.