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.
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.
Acceleration is not very important at a high enough level because you can mini-turbo up to top speed when you get hit.