C# compilation is fast in part because of the removal of headers and the replacement of them by well defined assemblies with interfaces. C# compilation is fast for the same reason Java compilation is fast. :)

C++ compilation is sort of screwy in that to compile a new C++ file the compiler actually has re-compile all the definitions of the libraries you are using. It is insanely inefficient compared to just using explicitly defined and immutable API interfaces on Assemblies/Modules.