And I'd love to peer in to some alternate universe that has an optimal Python interpreter (can't say "the" optimal because it's really a complicated frontier rather than a single point) that runs on our hardware and see what it looks like. Maybe even on multiple points on that frontier.

What really is the limit? I struggle to imagine what a C-speed Python interpreter could even look like, but is there some conceivable program that runs it at, say, half the speed? What even is the limit? What techniques would such a program use that would surprise us and be new and perhaps useful other places?

Or are actually pretty close to that frontier now?

To be honest, given the way optimizations tend to work, the answer probably is that we are relatively close today. They tend to have diminishing returns.

But I don't know. Is there some execution model nobody's thought of, or that has been thought of but simply hasn't had the effort invested to make it pay off, that would make huge gains? I can't prove there isn't. (In fact it's not far off junior-level computer science to prove that you can't prove it.)

I also suspect that we are relatively close, and there are diminishing returns. I suspect that you would have to start the language design with this goal in mind, and then balance out performance concerns with certain features.