Yeah, I never understood Zelda's speed either. Okay, she's strong and a lot of her moves have good knockback, so there's reason to slow her a bit, but slower than Bowser? Ganondorf? Mewtwo? It's absurd, a small human-like being should be far more nimble than a big, bulky, weighed down character.
I agree with Luthien though. They attempted making Zelda and Sheik interdependent, but also able to suffice on their own if they must. The problem was they gave one of those attributes to Zelda and the other to Sheik, not both to each. That's why Zelda's slow and unable to easily land many high-knockback attacks, and it's why Sheik is fast and her knockback isn't terrible. If Sheik's out then they will have most definitely boosted almost every aspect of Zelda.
Anyways, I guess a spellcaster could be far different than Zelda. The whole "hands and feet glow" thing is what makes her a physical fighter who just incorporates magic, whereas another spellcaster could be ranged. However, a ranged spellcaster should be lighter than Zelda and weaker, since the range would provide a safe area. Zelda shouldn't be a featherweight when she has less range than characters like Bowser.