> But an infix transform does delay the evaluation of expressions you've passed to it. Eg (infix (avg b) * (sgn a)).

It doesn't 'delay' anything. It just rewrites at compile time

   (infix (avg b) * (sgn a))

into

   (* (avg b) (sgn a))

That's all. At runtime only the rewritten statement will be executed.

> At a very low level, macros operate by having the evaluation of their arguments delayed

Not at all. The main purpose of macros is to compute code from arbitrary expressions at compile time. There is no 'delay' -> it's mostly about rewriting expressions at compile time.

> implementing a metacircular evaluator and learning when and how to perform evaluation.

You can build a macro expansion phase into an interpreter. That's what Lisp interpreters do. But the main purpose of macros is to enable code transformations at compile time and thus only to have a cost at compile time, not at runtime. In an interpreter the macro expansions would also happen at runtime and have a cost there. But Lisp compilers are popular and to make the most out of code transformations at compile time, the idea of macro and their expansion at compile-time was introduced. Thus one gets for example new embedded languages, without the need to parse them at runtime.