From a gameplay standpoint, Haste simply reduces attack delay, weaponskill use, and spell casting. In the Palace of the Dead, should the floor give the party Haste, it says something like "Ancient enchantments stimulate your humors, allowing you to act more quickly".
Now one can easily say that for physical attacks, you simply hit/attack faster because you got better reflexes. For magic....I have no idea! Anyone care to share a guess?