C# is not always slower - in many cases, it can perform just as well as any language you listed. Usually the algorithm has more to do with the speed than the choice of language.
However, C# is very expressive, and has a great base class library to work with, and super-fast compilation. This means that it's very easy to work with, and can allow you to be much more productive than many other languages, especially C++. For example, I just had a small project that I would have budgeted 1 man-week for in C++, and we finished it up in less than a day in C#, mainly because the base class libraries simplified so many of the tasks.
