The most important things are already said here. I would add:
The developer utilizes a language which the compiler(s) utilize(s) to generate machine instructions which the processor(s) utilize(s) to use system resources. A program will be "fast" when ALL parts of the chain perform optimally.
So for the "best" language choice:
- take that language which you are best able to control and
- which is able to instruct the compiler sufficiently to
- generate nearly optimal machine code so that
- the processor on the target machine is able to utilize processing resources optimally.
If you are not a performance expert you will have a hard time to archieve 'peak performance' within ANY language. Possibly C++ still provides the most options to control the machine instructions (especially SSE extensions a.s.o).
I suggest to orient on the well known 80:20 rule. This is fairly well true for all: the hardware, the languages/platforms and the developer efforts.
Developers have always relied on the hardware to fix all performance issues automatically due to an upgrade to a faster processor f.e.. What might have worked in the past will not work in the (nearest) future. The developer now has the responsibility to structure her programs accordingly for parallelized execution. Languages for virtual machines and virtual runtime environments will show some advantage here. And even without massive parallelization there is little to no reason why C# or Java shouldn't succeed similar well as C++.
@Edit: See this comparison of C#, Matlab and FORTRAN, where FORTRAN does not win alone!
