Forgot to post a further update, sorry:
The slowdowns have already been fixed in one of the previous releases, apparently
They only happen very sporadically now, and require no reload so this issue might as well be closed.
Thanks for catching up, though!

On nearly _all_ game saves, both automatic or manual, the game becomes noticably slower:
Debug view shows UPS goes down to 30 from 60 (FPS remains fixed 30).

Workaround: What does help is switching to map view with 'm', after which UPS returns to 60.
In rare cases reloading the savegame is ...