Go through every doughnut possible Try not to bonk on anything. (Except balloons.)
Those tips are obvious, however, there are some that are not.
The Egg is actually the fastest Stone available The more you move around, the slower you fall (air resistance?) so try moving as little as possible. Sometimes, going through a doughnut can lose time due to the time loss caused by moving to it. This usually isn't the case. Here is a a cool idea for you to experiment with if you want to.
Interestingly, if you are going fast enough (and I think some rng comes into play here) then you can clip through an object without losing any perceivable speed. This makes it possible to clip through a doughnut but still get its speed boost. This saves time because instead of moving over the center of the doughnut, you only need to get as far as the edges of it. This could also potentially allow for some big timesaves by clipping right through obstacles instead of maneuvering around them.