The way you can nest conditions in sed, like /foo/ { /bar/x }, is pretty powerful, and I knew a good bit of sed before I realized you could do that. It's not readily obvious from the manual.
It will be a challenge to make an interpreter for a better language faster than dead slow.
I would bet on a compiler for a better language instead. That way, only one person would have to wait ages and ages. I doubt the result would be fast, but it wouldn't be insanely, insanely, insanely slow, either.
I would much rather hand assemble machine code than program in sed. I imagine most people have a better understanding of how computer architecture works than how to do arithmetic and logic with sed.