Benchmarks are tricky and hard to find for dlang as its not that popular, but I have common sense to back that statement. Dlang garbage collector is really really slow for now [0], and C# has a very fast GC and overall is almost as fast as zero-overhead languages like Rust or C++ [1]. So, its reasonable that for anything that involves GC (we all can agree that web server does) D is not that fast as manual memory management languages yet.