In fact, multiple dispatch is essential to get good performance, abstract types have a cost if explicitly used inside struct definitions, but that's why you use parametric types, and abstract types do not have a cost in function signatures.

Macros are expanded at compile-time, and at least have no runtime cost, in fact they are often used for improved performance.

I think there's some fundamental misunderstanding going on here, but I'm not sure what it is. Or do you mean to say that if it is possible to write slow code in a language, then only a subset of that language has good performance? If that is the case, I don't know how to respond.

In fact, those aren't special features and there is no other subset. Those are the core abstractions on which everything in Julia, down to primitive types and wrappers for llvm intrinsics are built. Without them you wouldn't have Julia.

Julia's GPU ecosystem wouldn't be where it is with just one or two people maintaining it without being able to reuse those features to plug into existing machinery.