she has a ton more MS than the average champion. She gains the speed on any ability hit (not just Q), gains it on hitting anything (even jungle camps), and the speed stacks. So when she's in combat, she fast.
Oooh so that is the playstyle. That does seem interesting. Why isnt MS the main passive, and damage passive tied to an ability? The MS now gets overlooked. Would also allow her to scale over the entire game, matching gained MS of others in game from dead man or Zeal items. Especially given she already has damage on all her abilities, so her damage scales through entire game and the passive is %max HP.
The movement speed scales with Q rank so she hits a more powerful midgame spike.