Just to give you a bit of advice real quick:
-Try not to turn when you are not in a slide. It costs quite a bit of time.
-Try to take corners a bit tighter.
-On the 1lap attempt, you basically wasted your first shroom.
-Your third shroom could have cut more grass.
Also, you should try to use all three shrooms through the second set of grass, that is the fastest method for the 1lap.
Not that I am a good player, far from it, but I know the basics of the game.
Good luck improving.