It has to do with how the engine aka RenPy works with the hardware. One PC game about 8 years ago I had to cut my screen refresh from 120Hz to 60Hz and cut some other stuff... otherwise it would not run properly and kept lagging.It certainly isn't a question of PC performance as such, that would have been found out by now. But my pretty old off-the-shelf computer does not register any lag.
Edit: check your screen refresh and change it to 60Hz for the game and see what happens.....
Edit2: I tried didn't do anything *sigh*