Thanks!
Had not played Castle Bubble before I made this game but I did play other sylvie games.
I think the issue with the slowdown is not one of timing (or if it is it's very slight), but instead one of input polling. The game still runs at 60 fps even when slowed down, so the player moves with more granularity and thus has more "chances" to input a turn before they reach a certain point.
Due to the limitations of pico8 I'm not interested in solving this problem nor do I particularly mind if the player finds a way to cheese some of the challenges by slowing the game down.
Some obstacles are definitely too hard and finnicky -- this fact was not lost on me when designing the game.