I think C# is in the category of "fast enough".

When I was working through project Euler (http://projecteuler.net/) with various languages, the only one I really had to optimize was Python (vs C/C#/Haskell).

I really like Python, and for most stuff it's fast enough. Just saying the real penalty (that I noticed) is a scripting language, _if_ it's heavily arithmetically constrained.

Some (non-trivial) games in C#: http://www.arcengames.com/w/index.php/aiwar-alienbundle

http://murudai.com/solar/

However, it's possible and even practical to also write games in Python. See the pygame project.