Users running their PC in Dual Display configuration with a NVIDIA 6800 card will face low FPS and drops down to 3-4FPS in some areas of Doom III. But there's a way to fix it.
We've discovered a driver bug with NVIDIAs current drivers v61.76/61x77 in Doom III (we did not test with older drivers yet). It causes a performance drops down to 3FPS in certain parts of a level and a lower overall FPS, when your GeForce 6800 is running with multiple displays in dual view mode - and only on systems running in dual display mode!! If you have only one display attached, everything should be fine! We did not test yet if other cards are also affected.
If this happens, it can be fixed by setting the Hardware Acceleration mode from "Multi-display performance mode" (the default driver setting when dual display mode is used) to "Single Display Mode". You can find this in the "Performance & Quality" settings, under advanced settings.
NVIDIA is investigating the issue and working on it.