Maybe this quote from "Some Work on Arc" [1] is what you were thinking of:

> For example, Arc currently has first class macros. It just seems to be the simplest way to define the language. First-class macros are potentially very inefficient; you could be expanding macro calls at runtime. But I think most users won't want to take advantage of this possibility. They'll ordinarily make some kind of global declaration that macro calls can all be expanded at compile time, and macros won't cost any more than they do in current Lisps.