At the beginning of this video, you'll see it takes me exactly 9.85 seconds to get from one end of the beach to the sandcastle.

Next, what you want to do is go into Sky World (you can glitch inside the same way you glitch inside the boss' lair if you don't have enough stars) and head to the left.

What this blue powerup is supposed to do is give you flight which would kill you since there's lava right above you. Instead, we teleport to spawn and wait for the cooldown.

You see my time from one end of the beach to the other went from 9.85 seconds to 7.77 seconds. This difference will obviously increase in longer runs.

Credit to SnivyGames for originally showing me this.