[carapace]

> Is there something about the primitives that APL-derivitives provide that mean that more of your program can happen inside the primitives so the interpreter is doing less work?

I'm not an {APL,K,J} expert by any definition, but FWIW I think that's the "array-oriented" part of it: syntactically the operators don't specify the "rank" of their args so the interpreter is free to select the most efficient low-level semantics. Something like that.