Tomb Raider Special - lag in last level? (PC)

I have an older computer, so I have my graphics quality pre-set to LOW, which works fine in most levels. The first three Tomb Raider Special levels were going well, and then I hit the Treasure Room and my cursor was lagging all over the place.

Any idea which graphics options I should change? Is it the shadows? What is causing this level to lag so badly?