I've found a couple of boots of speed, which of course gives me permanent haste. I've noticed that my cleric now casts spells a lot faster (he's almost done with them before he can say the magic words :D ), and I wonder if the haste spell should really do this. I thought it was just for faster movement.

it gives an extra partial action that you can either use for:

movement
casting an extra spell
making an extra attack at highest attack