> C, D, C++, rust, and nim should all have comparable performance; they all go through gcc|llvm, and all have the same optimizations.

That's not necessarily true.

For example language semantics (essentially how much information the optimiser has) such as whether mutable pointers are allowed to alias (or even the existence of non-mutable pointers) can play a huge role in what optimisations can be applied in a given situation and even how effective certain optimisations are. Also default behaviours like D, rust, and nim doing bounds-check can change the practical speed (although not necessarily theoretical speed) of a language. And finally things like garbage collectors can make certain languages faster in some cases but slower in others (again there's a practical vs theoretical argument here, since you could get similar speed-ups using other techniques).

So no they don't necessarily have to have comparable performance, at least not in all situations.