Ah I found my mistake! I am playing on Vice and I had it set to NTSC, which seems to get 30 fps. When I switch to PAL I get the full 50 fps! Thanks for creating such a cheerful, if difficult, game :D
I played iim and it looks like it's scrolling at 25 fps, but the preview videos for iio are clearly scrolling at the full 50 fps. Is this some new scrolling tech in iio?