While turning it's faster to tap really fast instead of holding (youll see that on fullspeed tracks a lot)
Pressing brake in the air is very important to not lose speed
It's faster to drive on the ground than fly (so try to reduce airtime)
This isn't everything of course and there are hundreds of tiny things to learn and every track is different, so look at the wr on community tracks and the official tracks so see what's the fastest way to drive certain parts of a track.
hope this helps :)