There's two big tricks in this game, that are easy to implement and save a lot of time.

Invincibility Glitch (Credit goes to Birds Probably)

If you "Retry" a stage at the same time as you are hit by an enemy, you are invincible for the rest of the run, making the Spikey stages in particular much faster. Easiest done with the very first snail on the right wall in Snail Spot.

Fast Kiwi Cutscene (Credits go to elwiwi)

It can seem kinda random, how fast the Kiwi cutscene before the last level of any% can go. The fastest way to get through them is to PRESS AND HOLD A/B/ESC/ENTER before the text boxes appear in each scene.