Why is this D code slower than C++?

[Bradley Smith]

%u wrote:
> Bill Baxter Wrote:
>> So the C++ code is ok.  But it's not clear why Material became a
>> class in the D version rather than a struct.
> Thx. I did not notice, that "Material" is a struct in the cpp-version.
> 
> This shows however, that programmers still are not following engeering principles: no technical documentation of the port is given and no one complains.
> 
> Instead several people are eager searching flaws in the reference implementation of D for which there is also no technical documentation :-(

What technical documentation would be proper? What would it contain?