Basically the problems are as you describe, terrible and inconsistent performance. Low frame rates when enemies appear.
The game just runs badly, although there is little consensus on whether the game or nVidia's drivers are at fault. There's a tendency to suggest that it's the game because AMD users are having similar issues.
In terms of the problems themselves, the SLI profile doesn't work properly, MSAA in game is broken and shadows result in excessive CPU load.
Various solutions have been found to help.
Load the custom SLI profile that's been knocking around (google it). Load it in NVInspector.
Disabling SLI results in more consistent, although not better performance.
Turn off in game MSAA and over-ride the game's anti-aliasing from within NV control panel.
Turn shadows down to medium.
Write to Ubi Soft and tell them that you don't appreciate them launching games with beta code.