> Saying that one isa is faster or more energy efficient is like saying that c++ syntax is faster than java syntax.

I think that could be a valid statement. APIs can influence performance by constraining the implementation. For instance, the syntax for constructing an object in C++ will, generally speaking, always yield faster code than Java, because Java objects are almost always allocated on the heap, while C++ objects can be allocated on the stack. Compare:

  // C++
  MyObj o{};

  // vs. Java
  MyObj o = new MyObj();

Sure, it's possible to write a Java allocator/GC that will yield similar performance to the C++ code, but in general, that will practically never be the case. The syntax of the language has constrained the implementation so that Java will practically always be slower. Presumably, similar design choices in an ISA could have the same effect.