Why should turboprop rpm be half that of a piston engine on a comparable type, say DC3 against DH8C.
it is not. the dc3 has 16/9 reduction gearing so 2000 engine rpm are far not 2000 prop rpm.
i think the extreme was a lockheed starliner where the engine turned the props via a .355 gearing so prop speeds were something about 750-800rpm in cruise .