This game is also available on Wii.
Before the end of the lap you should be driving over a bridge into a cave. While in the cave you should see three item boxes in a row. Turn your Kart until you drive through all three of them, then hold Left (or Right for mirrored courses). If done correctly, you should drive right up the hill and cut about five to seven seconds off your time.
The second short-cut can be used if you have a Triple-Mushroom. After crossing the final bridge, instead of heading up the path to the finish line, turn into the grass on the left, and keep near the fence. Use the first mushroom just as you leave the path. Use the other two mushrooms, each after the previous one is dying. Although you are far to the left of the finish line, it will count every time, and you will end up at the gate entrance to the maze again, eliminating more track and time from your race.
The Lakitu will pick you up and put you in one of three places: (1) inside the cave, down near the bridge entrance, thus granting you 3/4 of the lap. (2) behind the finish line just at the exit of the cave (granting you nothing). (3) at the first turn in the course, up past the start line (granting you nothing).
You want to be put at (1) every time. If you are put at (2) or (3), it is because you weren't vearing far enough to the left, away from the finish line. Once at (1), go through the cave, and repeat the jump. The second time, you will complete lap 1 as you are falling toward the water. Once you're at (1) on your final lap, you do not want to do the jump again, just go through the cave exit, and cross the finish line. If done correctly this will give you the following aproximate lap times: 25, 15, 15.
Before the end of the lap you should be driving over a bridge into a cave. While in the cave you should see three item boxes in a row. Turn your Kart until you drive through all three of them, then hold Left (or Right for mirrored courses). If done correctly, you should drive right up the hill and cut about five to seven seconds off your time.