What is causing the lag?

Heroes 5, original, licensed version. When playing the campaign, specifically the mission for the queen, in the "Trap" map, there's a noticeable lag in the middle area where the demons are, their territory. When looking at that area, the game lags significantly, and the hero moves with severe lag. However, if you shift the view to the forest, everything runs smoothly. So, the lag seems to be specific to that "area." :confused:
Specs:
Amd 4000+
GeForce 8600GT
2GB RAM
:) Help me figure this out.