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.

[0] https://olshansky.me//gc/runtime/dlang/2017/06/14/inside-d-g...

[1] http://benchmarksgame.alioth.debian.org/u64q/binarytrees.htm...