Speedruns are for playing the track solo, right?
If you're playing normally (with other people and items), even the most skilled player can't avoid getting hit by some universal items and will benefit from acceleration.
Hence, as the author notes, the balanced build with Peach being favored among competitive players.
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GP first link is to a speedrun in the all cups category, played with items and computer opponents. There's a video included.
People do speedrun the individual tracks solo in other categories.
For a speedrun you don't care about 100 failed runs as long as you get the world record once. You're basically hoping to get lucky that the computer opponent won't hit you. Obviously you're going to pick the greediest (i.e highest speed) choice in that setting, that doesn't make it good in normal gameplay.
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It's a fair point that such an environment will be different than playing against other humans