All right, I'll bite, how fast is Kenpachi? The bells in his hair thing points to him being subsonic, though he has apparently got some pretty good reaction speed. The issues I see here are:
-In character Rainbow Dash is likely to just leave/avoid a fight if Kenpachi upsets her, unless something is on the line. Though she is generally the most aggressive and quickest to anger of her friends, if she did confront him she'd be unlikely to back down. She'd also cheat.
-Rainbow Dash can fly. As far as being caught, it's out of the question unless he's very fast. He might be able to airwalk after her, but he's never been seen doing so. shrug Even assuming he can, it'd be hard to compete with her natural flight, she's extremely talented.
If all she needs to do is avoid being captured, this is kind of rigged.
-If she's actually required to fight, I'm not sure how much it would take to get her serious. She seemed to have no compunctions about fighting changelings, if she considers a human on the same level as a monster like a dragon, since she's never seen one before, she could go straight for the ass beating.
YouTube video She's able to dodge lightning with no PIS involved.
http://www.youtube.com/watch?v=CKvDNAygDlY&t=3m8s
As a filly she shook all of Canterlot with her first Sonic Rainboom, and current Rainbow Dash can Rainboom on command, and can even survive flying that fast directly into the ground as seen here:
YouTube video
Her first rainboom also could crack open rocks miles away:
http://www.youtube.com/watch?v=4iolsBLzo28&t=20s
And caused hurricane force winds from further than that:
http://www.youtube.com/watch?v=hMlclV0Tmkc&t=17s
She's a blackbelt in karate, and can do cool things even without her superspeed. For example:
http://www.youtube.com/watch?v=m-Y6hJj0ZOI&t=35s
She sends a tower of rocks flying, shattering some. She does this with her head because she's a bawss.
http://www.youtube.com/watch?v=P6rB9W3VG2Q
She can casually kick through whole trees and things.
http://www.youtube.com/watch?v=YLD91NpzPQM&t=12m30s
Cause tornadoes that force her friends to hold onto trees to avoid being blown away. This is notably without her rainboom speed, Swarm of the Century occurs before Sonic Rainboom.
The physics bronies have calc'd her at all kinds of crazy speeds, but she's clearly very fast, strong, and durable.