It wasn't from dual screen. It just randomly hit 32-37 fps at different times of playing through the part where you first come into the Niwa area.
I can deal with it, it's just annoying because my youtube playthrough looks crappy with the sudden frame fluctuations, but I'm a nobody on YT anyway so it's all good.
That is sot of dual screen, as well, in that it flashes the full screen image between the real, and spirit worlds, so is still taking the same sort of two game rendering hit to the FPS.
My advice is this slow paced game is better played at a steady locked 30 FPS, if you can't maintain a steady locked 60 FPS.
Recording the games, via OBS, capture card etc. is going to impact performance as well, unless you have a 2nd PC for that.