Ok I did a benchmark (RayTraceBenchmark):https://github.com/zezba9000/RayTraceBenchmark (Feel free to do pull-requests to add more langs or post up ports [would like to see the results])

It renders a 3D scene and saves a RAW image file. You can open the image via Photoshop or whatever. The resolution of the image is 1280x720.

Here are the current resolts from the test:https://github.com/zezba9000/RayTraceBenchmark/blob/master/C%23/Results.txt

As you can see Linux/BSD run slower on the same computer? This should not happen?

<<< AMD Athlon 64 X2 Dual Core 4600+ 2.40GHz >>>
.NET 4.5 (Win8-Win32)
(x86) = 1.179 sec
(x64) = 1.549 sec

Mono 3.2.3 (Win8-Win32)
(x86) = 2.059 sec
(x64) = 2.07 sec

Mono 3.2.3 (PC-Linux)
(x86) = 2.425
(x64) = 2.409

Mono 3.2.1 (PC-BSD)
(x86) = 2.536
(x64) = 2.509

<<< Intel Core2 Duo P8600 2.40GHz >>>
.NET 4.5 (Win7-Win32)
(x86) = 1.05 sec
(x64) = 1.132 sec

Mono 3.2.3 (Win7-Win32)
(x86) = 1.692 sec
(x64) = 1.702 sec

Mono 3.2.3 (OSX 10.9)
(x86) = 1.675 sec
(x64) = 1.679 sec

As C# is developed by Microsoft as part of .NET, these results aren't strange at all.

Microsoft simple has the resources to build the best compiler (runtime performance in this case).

Well I think you missed how Mono(not .NET) on OSX/Win ran faster. Which doesn't make sense. There is a performance bug in Mono me thinks :)