Metal is probably the easiest graphical API to work with at the moment. It also includes some really interesting tools that no other platform offer AFAIK. That said, Apple's reluctance to dedicate more resources to Metal and the fact that few eyes after its introduction it still has some glaring omissions, makes me question the entire enterprise. I am afraid that Apple has lost the momentum here. Metal is certainly mature enough for a lot of applications and its an attractive target for mobile gaming, but the situation on desktop looks not that nice.
As to performance: modern APIs are all about efficiency. That is, their goal is to allow you to write code that doesn't slow you down artificially. There is always some sort of communication and setup overhead when managing hardware resources (here: GPU), and modern APIs attempt to bring that overhead down. In an ideal state, the performance should be identical. However, DX12, Vulkan and Metal make different choices. Metal focuses on convenience, which usually means more potential overhead. Vulkan on the other head focuses on pure efficiency, but its also very hard to work with.
Finally, the main reason why games run faster on Windows is simply because Windows is a popular gaming platform (that generates a LOT OF money) and OS X is not. Hardware vendors generate a lot of their revenue from selling gaming GPUs. This has lead to an curious situation where Windows graphical drivers consist mainly of various hacks that speed up popular games. Have you ever wondered why a windows GPU driver is so large? Well, thats the main reason — it contains a lot of rewritten shaders and specialised driver code to make games run faster. This is ridiculous, but it "works" and most importantly, it makes money. Interestingly enough, the new APIs will probably improve this situation as they will make a lot (but not all) of these driver hacks unnecessary.
P.S. I am currently developing a small simulation game in my free time. I use Metal on OS X and try to use Vulkan on Linux/Windows (but I didn't do much for the non-Mac version yet).