I agree that the whole point of generators is performance. When combined with lazy sequences (srfi-127), it covers many typical use cases of streams much more efficiently.
I've written Gauche version with performance in mind, but it depends on a few non-standard features and can not be used as-is. Besides, if we do go for performance, we need benchmark suites as well. I'm interested in adopting Gauche version as the reference implementation but don't know when I have time. If you have some code to improve the current reference implementation and can merge it to the repo, we can proceed gradually, I guess.