Using deltaTime in Update. But that wasn't the issue. It was just a weird Unity physics thing. I just changed it so I manually calculate the boost speed based on the slope angle instead of letting Unity do whatever it was doing, and it works fine now.
Ah, gotcha. To be fair, physics stuff is usually better off using fixed time for this very reason. This article is really helpful in understanding; maybe it'll be useful to you. The article discusses this from the angle of a different issue, but applying physics using regular deltaTime is impacted by framerate.
Great work, by the way, super creative!