[Morgeruat]

typically lower level spells are faster, in teh spell description of each spell it lists a casting time, this is how long it takes the spellcaster to do his thing to get the spell to work, either by shaping the arcane magicks or by calling on the might of his god.
if you have the bonus cd, or downloaded the goodies off of it, can't remember where you can get it, one of the merchants in the adventurers mart has the robe of Vecna, which lowers casting time by 4 or 5, there is also an amulet you get in act 3 that lowers casting time by 1