D is indeed interesting. Wrote some experimental code this fall, was fairly happy with it, but wound up going w/ rust.
On the other hand, D supports a lot of great C++-style stuff, but I'd never pick it in that regime because it'd be the same amount of effort without as-fast-as-possible.
It is also possible for D to be as fast as C++ in many domains: we have LDC based on LLVM for optimisation (parity with clang), but I think that one of the biggest things is the flexibility to have (and determine what is) an efficient architecture.
While the first part is perhaps somewhat subjective, the second is just wrong. D allows you to get to the same bare-metal, no-holds-barred level of performance that C++ does – I wonder where that misconception would come from.
Note that this is not just an academic possibility, but there are real-world use(r)s of D in exactly that domain, for example the folks at Weka.IO for their distributed file system. Of course, their sub-millisecond latencies don't leave much room for careless use of the garbage collector. But being careful about memory allocations for that sort of application is just as sensible a thing to do in D as it is in C++.