Which language, and why? (C++/C)

This question does not make any sense at all. A computer language cannot be measured in milliseconds.

If this is not quite apparent to you, your claim about your good knowledge of C# sounds more than suspicious. And, in my opinion, most people knowing only one computer language cannot know it well, just because it often stimulated very narrow and inflexible thinking. Of course, this is not a rule, but looks like a really working rule of thumb. So, your idea to learn something else is quite productive.

Now, your idea about limitation of .NET framework is pretty much wrong. .NET can be considered as yet another platform, not much different from Windows (CLR does not have to be limited by Windows). It is pre-installed on newer versions of Windows, so your problem of limitation is a bit artificial. Yes, the user base is limited; it will exclude some users (loosers :-)). But with Windows API, you exclude, say, Linux users (but you can develop for both Windows and Linux using .NET, as well as some other platforms, including Mac OS X, for your information). Effectively, .NET can give you wider, not narrower user base. And I never heard of any serious problems with .NET performance.

Don't get me wrong: I am not trying to insist on .NET. But I would rather advise something really different, based on very different paradigms and not that archaic as C/C++. For example, Haskell, Closure, Python, Scala, F#, even Prolog (I am listing them without much thinking, just for example).

Remember:

Alan Perlis told us:

A language that doesn't affect the way you think about programming, is not worth knowing.
http://www.cs.yale.edu/quotes.html

Golden words!

—SA