It is not this project' s goal to replace the native compiler. If you think performance may be an issue, you always have the native compiler option.
However, I spent enough time in the industry to buy off the bat statements such as "language (or platform) A is inherently slower than B". Remember the old days when people thought is not even worth considering C++ because dispatching method calls
through virtual tables was a performance killer? Well, some people may still swear by that today.
My take is that unless you prove your point with profiling data and benchmarks I do not believe you that D programs compiled for .net are significantly slower than native code.
Remember that in .NET what eventually runs on the hardware is native code, compiled from IL into machine code by the Just In Time engine. The executables are cached, so the JIT-ing costs tends to be amortized over time.
With no D compiler to generate IL code you can't profile and compare D on .NET versus D on the native OS. Oh wait, now you can!