I do not particularly like this situation (I'd prefer to use Python on all platforms) but until this changes, C# is the only way to support all platforms from a common codebase.
As far as performance is concerned... unless you are an AAA studio with tons of funding, you won't be able to take advantage of the extra performance of C++ over C#. Why? Because (a) the most important concern will be getting your product into a shipping shape, and (b) micro-optimizing in C++ is the last thing you should be doing while trying to ship a product in time. Unoptimized C++ is really not the most performant thing in the world and it is very easy to blow performance there (and I have seen this time and time again by non-expert programmers).
Wringing this extra 10%-50% performance out of C++ can take time you simply don't have. Hell, even AAA studios release unoptimized messes to ship in time, nowadays, so it's not as if the choice of language is the deciding performance factor anymore.