Dash dancing is used for making opportunities...usually for a grab. With Peach...if combined with her horrid Wave Dash, it can give the illusion that she's a little faster than she is, xD. Practice your WD out of Dash, and dash dancing right back into it. Slikvik once showed me this. He said to get used to moving like that at all times, because that's about it. Peach can't really move much faster than that, xD. It's true, she's a slow character. Peach should always be moving if at all possible, and also she should have a threat of turnip..when needed. It's so you can be offensive AND defensive at the same time.
I was watching JFox in the crew battle and he was VERY active. There wasn't much of a time when he wasn't moving, which kind of puts your opponent on edge. If you have a turnip in hand it makes them even more wary of you. With a turnip, you can still get a FC aerial off, and then follow up with the turnip. Just remember your options, and practice Peach's movements. She looks fast because the really good Peach players are always moving. DD->WD->DD. Throw a Bair, Nair, or retreating Fair in there as well and then continue.
I'm sure someone will expound a bit more.