I've always been under the impression that slower weapons received more speed bonus (Wider spectrum, also more likely to get huge negative values as well). On the other hand, I don't have any numbers after original M&B, but I do recall having 600+% speed bonus with the great lance frequently. There was a clear trend with slower weapon and high speed bonuses. In Warband I've mostly played multiplayer but it has certainly felt like it. Can't say for sure without numbers.