Heh i got same problem.
The most lag i got was from the terrain for me due to using that program that made my map 480x480.
I can give you a tip.
If it lags when you use Terrain Deforming spells like Shockwave or such just make a spell like carrion swarm and replace the damge mana and all the other stuff with the spell you want.
That way it won't deform the terrain and won't lag
Hope it helps.