Random Slowdown?

[bkudaimi]

When running F-Zero GX on Dolphin 4.0 x64, my PC does well, except for one map called Sand Ocean Surface Slide. When starting this map, the speed starts at 28-32% instead of 100% and doesn't get better at all through gameplay. I don't know why it slows down on this one level and nothing else. I use a Core i7-4712HQ @2.30 GHz, 16 GB DDR3, and an Nvidia GeForce GT 750M (Dolphin 5.0 didn't run the game well, that's why I use 4.0). Any help is greatly appreciated!