[color:"orange"]I'll try the sound off (although sound has given me no problems in the first act).[/color]
I was thinking the problem might be with sound effects for the mutants. Since the problem is with mutants mixing with other opponents, that implies something relating to the mutants is causing a conflict. Since you've already tried a clean re-install, it is probably not a corrupt file, unless there is a scratch/defect in one of the CDs or your optical drive has a problem reading one of them.
If the problem isn't graphics or sound related, it could have something to do with enemy AI. Maybe try learning the tracking skill (increases the range you can see NPCs on the mini-map) and see if that helps or makes the problem worse. One of the merchants in the imp village teaches Tracking, as does Annabelle in the BF.
[color:"orange"]The problem seems to reappear as soon as I reach the lava fields. It usually just slows down (very much) until I go to the battlefields.[/color]
Were there mutants in the lava fields? Fire elementals and demons were the main opponents, but I don't recall any mutants.
After you rescued Tax and the chieftain's son, there was a cut scene as you crossed the lava with Samuel sending a hunter after you in the form of a tibar, correct? The first time you entered the lava field, did you get a cut scene of that tibar changing back to its original form and attacking you? In an unpatched game it was possible to find the lava path yourself, though that could cause hanging cut scenes, rather than slowdowns.
Do you have access to another computer you could try the game on? If the same thing happens on a different computer, the problem may be something in the save game folder itself. If a save works elsewhere, then you know it is the game itself on the laptop, or a conflict with something else in that system.
