From following their issue tracker: some ideas didn't pan out (yet), some ideas turned out to break backwards compatibility, several pyperformance tests broke because of the changes made so they had to fix upstream (eg gevent and cython) and a lot of the work seems to be setting up future optimizations. They're also getting their changes merged directly into CPython main, so I reckon there's more red tape than if they had done the work on a fork. It does seem to inspire fellow core devs to pitch in and try some of their own ideas, so I can see the effort pick up steam as they make more headway.