Speedrunning this game is very different than playing it casually. It heavily relies on the assist mode that was intended for multiple players, but for this run, you're doing it all by yourself! Ideally you'll have four wii remotes connected and ready when you play, but if you don't have four wii remotes, you can still make do with only 1 or 2.

It's generally preferable to collect fruit with the wii remote instead of with Yoshi. When Yoshi eats a fruit, he will temporarily stop in place, which does not happen if you collect fruit with the wii remote. Additionally, every time Yoshi eats a fruit, if it fills his fuel gauge past the max, he will lay an egg. Each egg you lay will play a firework animation at the end of the level that loses time. Ending levels with low amounts of fuel will let you eat fruit as Yoshi without laying eggs (which is useful for stages with more than 4 fruit, as Yoshi will have to eat something).

If you collect the final fruit with the wii remote on the frame before Yoshi bumps into the exit gate, you can effectively skip the "gate opening" cutscene. This is an extremely precise trick, but considering it saves 3 seconds every time you pull it off, this glitch could potentially save up to one minute in the 20 levels category. Here's a video showing off the glitch:

The rules are a bit ambiguous about this, but technically speaking, if you intentionally game over on level 1, then go to "player setup," you can select a Mii and hit start without watching the "Mii transformation" cutscene, saving a little time.