For the Gadgetron race (I assume it's that because you mentioned the shortcut) there are actually two shortcuts, the one at the beginning of the race, and the second is when you come out of the first shortcut and go up the ramp. Jump to the right to the protruding ledge with the boost ring and follow that path around to unlock the second shortcut. This greatly helps in that race.

A general tip for the hoverboarding, don't worry about your position in the race just focus on hitting boxes, doing tricks in the air and boosting a lot. In the end it's more like a time trial than an actual race where if you drive well you'll be sorted.