From my experience C# SOE are the fastest. It is also discussed here.

ArcGIS Server is developed in Java, how come .NET is the fastest?

  • Would you be able to quantify that experience in any way? If not, then I suspect that your question will be too broad and open to opinions because speed can be measured in many ways. By making your question about a specific test, which you describe, it will enable your question to qualify as focussed Q&A. – PolyGeo Feb 4 '16 at 23:25