Ars Technica took it upon themselves to benchmark Metal in the latest OSX El Capitan release. Even though OpenGL on Mac OSX is not considered to be on par with its Linux counterparts, which is probably due to the driver situation until recently, it pulls ahead of Metal in many situations.
Image Credit: Ars Technica
Unlike the other graphics APIs, Metal uses the traditional binding model. Basically, you have a GPU object that you attach your data to, then call one of a handful of “draw” functions to signal the driver. DirectX 12, Vulkan, and Mantle, on the other hand, treat work like commands on queues. The latter model works better in multi-core environments, and it aligns with GPU compute APIs, but the former is easier to port OpenGL and DirectX 11 applications to.
Ars Technica notes that faster GPUs, such as the NVIDIA GeForce GTX 680MX, show higher gains than slower ones. Their “best explanation” is that “faster GPUs can offload more work from the CPU”. That is pretty much true, yes. The new APIs are designed to keep GPUs loaded and working as much as possible, because they really do sit around doing nothing a lot. If you are able to keep a GPU loaded, because it can't accept much load in the first place, then there is little benefit to decreasing CPU load or spreading out across multiple cores.
Granted, there are many ways that benchmarks like these could be incorrectly used. I'll assume that Ars Technica and GFXBench are not making any simple mistakes, though, but it's good to be critical just in case.