Rosalina is the best in 3D World because default Rosalina is the best "power-up" in the game. I say this as someone who can speed run 3D World
Her low movement speed on the ground is completely irrelevant, because she can just long jump and spin everywhere, which is faster than Toad's running (the second fastest method of natural ground travel in 3D World).
The spin combined with the long jump takes Rosalina very far across pits too. It is also amazing if you can build up momentum through sliding on hills and jumping and spinning from them, and stuff like that. This allows her to just flat out skip a lot of obstacles, more so than Peach's float, and at a faster, more consistent rate since float does not work with the long jump and is very slow and doesn't go that far. Building up horizontal aerial momentum is not as easy with Peach either, because Rosalina has floatier physics and falls slower than Peach does.
However, outside of the obvious double jump and attack properties of the spin, it has a hidden property they don't tell you about - momentum shifting.
If Rosalina performs the spin, her momentum swiftly shifts in whatever direction the control stick is held. This can allow her to carry momentum in a straight line by continuing to hold the same direction she jumped towards, cancel all forward momentum by pulling in the opposite direction, or to make sharp mid-air turns to the left or right. This allows her to do things like infinitely wall jumping incredibly fast off a corner wall, even faster than scaling it with the cat suit:
She can also infinitely traverse horizontally across a flat wall, allowing her to bypass sections quickly that would normally require waiting for some kind of foothold to traverse (in this case, giant ants that walk slowly across the spikes that you need to bounce on) And of course, the different exploits can be seamlessly combined together:
It's also possible to make Rosalina infinitely scale VERTICALLY up a single wall, but these takes a lot of precision and practice and is only really useful if you want to get out of bounds. You never actually need long stretches of this for speed running, but short bursts of it can get her on top of platforms quicker than the others:
Combing techniques, you can pull of crazy things like this:
I feel like this hidden property is probably intentional, combined with her Luigi-like air physics, to reference her control over gravity.
As well as this, since the spin is an excellent attack too, you can basically ignore most enemy threats while you focus on the platforming. She's also the best in enemy rumble stages because of this.
As far as Peach goes, without any power-ups, she is the worst character in the game for speed running. She has a slow running speed, and doesn't have a way of making up for it with a technique that speeds her up like Rosalina (her long jump doesn't go as far because she falls faster, and she can't use the float out of long jump).
Peach CAN be the best speed running character, but ONLY in a select few levels, with power-ups. It also has the disadvantage of requiring pre-preparation. Essentially, you have to get a Tanooki Suit, and then get a Cat Suit to push the Tanooki Suit into your item reserve. Then, you have to find a really high area or a place that allows for an extreme speed boost, near the beginning of the stage (otherwise it's redundant). You then have to place the Tanooki Leaf precisely on the edge, and do the "Cat Suit Boost" trick into it (or use the other means of boost nearby - like using the "infinite acceleration" glitch on ice). You have to jump RIGHT before hitting the Tanooki Leaf. This allows you to transform mid-jump, and then you will retain momentum from the boost you performed. You can then go really far with Peach's float, and even further still with the Tanooki Suit float. If the level has a high starting point, and a low goal post, or an extreme boost of sorts at the start like the ice for the acceleration glitch, you can fly across the entire level, straight to the goal.
However, since this is only useful in certain levels, Peach cannot be called the best speed run character overall. Rosalina is better in any other level, and especially if you are factoring in collectibles.
In standard, fresh save file runs of SM3DW, people actually use Luigi for the majority of the playthrough, since his jump properties are like Rosalina's, just without the spin, and he runs faster than Peach. Then they switch out to Peach if they want to do the level skip trick in an applicable level, but this is actually not useful in a 100% run.
EDIT: Rosalina can also defeat the Histocratts in seconds because she can just back flip and spin on top of them, not needing to use the plates.
