I just bought Mario Kart 8 and was playing with friends. Someone said it seems like he goes a little slower when dragging an item behind his kart to defend against attacks. Was that an optical illusion, or do you slow down? I don't think previous games had a penalty for item dragging.
From my own experience with the game, and from some searching around Google and the like, I cannot find any evidence to support your question. As the game currently plays, holding an item behind you does not slow your Kart down.
Weirdly enough, the first result on Google when searching for an answer was a link to this question...