Are there any instructions that ask for/demand a pre-fetch of code from a *different* ebb?
Since the Forth data stack will almost certainly be in memory, the interpreter will likely often spend time loading operands from the data stack onto its own belt, before calling functions that implement Forth primitives. So if there’s a way to get code (e.g. for an upcoming primitive) fetched ahead of time, that would help speed up the interpreter loop considerably.
For a first-ever Mill interpreter, I’m more concerned with getting something to work correctly (with understandable code — a challenge in assembler!) than I am with optimal efficiency. That said, fast execution is certainly a plus.