Answer to the Op and based in this unity page http://blogs.unity3d.com/2014/05/20/the-future-of-scripting-in-unity/http://
C# runtime performance still lags behind C/C++
Garbage collection can cause pauses while running
C++ compilers and linkers provide a vast array of advanced optimisations previously unavailable.
Fundamentally, it differs from the current implementation in that the IL2CPP compiler converts assemblies into C++ source code. It then leverages the standard platform C++ compilers to produce native binaries.