Re: When and why did speed become less important?
Players in the current top 10 I'd call quick: Serena, Li, Radwanska, Halep, Jankovic, Kerber, and Cibulkova.
That's 7 players out of the top 10. Really I think you're just sort of singling out Sharapova, Azarenka, and Kvitova here. Sharapova isn't quick but she gets the most out of her movement with good footwork, ditto for Azarenka. Kvitova not so much, and you can argue that that's one reason among a few others for why she hasn't had success on the level of those two.
Speed and movement in general are definitely still quite important.