Most villains by their nature are slower than their hero counterparts. Metaknight seems to be a exception to this rule, but then again, people always fuss over his role now don't they? Anyways, as for the other pottential villains.
Ridley: Ridley will probably be fastish. He is skinny and has wings, so you'd think he'd be fast. For that matter you'd think he'd be light. Of course he could also be heavy and slow due to his nature, so it's a hard bet on him.
Dedede: He screams slow. Not as slow as say Bowser, but he'd still have to be slow. Maybe at around Wario's speed. (Wario's actually not that slow, it's just people think he is due to the apperance)
Wolf: I doubt Wolf's getting in at all, but if he did, he'd be slower than Fox. He could still be somewhat fast though...but I doubt anything beyond Link or Samus.
Mainly all the other villains have worse chances, and mostly are all slower than their heroes of their games. Seeing Sakurai's nostalgic take on Sonic, I really doubt he'd waste the third third party spot on Shadow. =/