I see no problem with heavy/slow. If someone stabs you in the leg with a spear, you're not just going to up and run away. If someone hits you with fire, you're going to just not run away. You're going to be on fire and burning to death and the pain is going to slow you down. Being hit with water is going to cause whatever you're wearing to get heavier as it takes on more water, causing weight, therefore slowing you down. The only exception is Wind spells.