Well, I looked at the battle and while there was some small amount of slowdown at times, it never got over 1 second a spell. I've no hotrod machine either, albeit 768 megs of ram and an NForce2 motherboard meaning fast memory access.
Also, given the way the computer casts spells once past the script, I hardly think it's dedicating any CPU cycles to something you could consider "AI".
I mean - casting spells at the militia that won't get there for 3 turns, as opposed to the heavy cavalry that arrives in 1, for instance, or casting spells that not only kill bodyguards and other friendly forces, but the spell caster himself when there were better (less costly in fatigue, doing more damage to the foe) spells to be cast.