Hey, I think "After the dialogue, everything is smooth as butter again." is a good clue that this is not a performance issue. In fact, while in dialogue, there's almost nothing to render out, and it's a relatively simple part of the game for the CPU.
This could be an issue with more than one application "fighting" over the Windows cursor capture, and probably an another software won, so CHORDOSIS got unfocused. that's why after the in-game cursor disappears, things go back to smooth.
if that happens, click outside the game and click back in, to make sure CHORDOSIS is focused. You can test this by just clicking outside the game any time and you'll receive about 8 fps.
Truly hope this helps, and hope you'll have fun with my game!