A region attachment is 4 vertex transforms, which use CPU. A mesh is 3 or more, so is only more efficient CPU-wise at 3 vertices, the same at 4, and less above 4.
All pixels affect fill rate, even transparent pixels. Using a mesh to avoid drawing pixels saves fill rate, but likely uses more CPU.
For most devices, you are more likely to run into a CPU bottleneck than fill rate. If your app is not limited by fill rate or CPU, then any time spent optimizing those things was time wasted. If you do have fill rate problems, it's easy to go back and change a region into a mesh to add vertices, so I suggest using region attachments to start.
