Throughout the game I usually get 50-60 fps on ultra settings with 4 AA. For some reason when I go to the sandstorm areas like Ivanpah Lake I drop down to 30-40. If I turn off AA my fps goes back to 60 in those places. Why is AA causing me to slow down in a sandstorm? I've already done performance tweaks like .ini settings, the d3d dll fix, and the 4gb patch.

Specs:

Ati 4850 with 10.10 catalyst drivers
AMD 955 quad core 3.2 ghz
4 gigs ram
windows 7 64 bit

Sand storms have lots of particles, meaning lots of things to process for the GPU.

That being said, 40 FPS or better is smooth for this game. So, even though your FPS is dropping, it is not to a level that is going to cause you issues in the game. Unless you keep the FPS counter going while you play.

As the mod said sandstorms involve a large number of particle effects. These are particle and cloud effects are smoothed by AA so as not to look pixelated- so they represent quite alot of work for your graphics card in and of themselves.

A dip in framerate (to what is still a very playable level) is not unexpected in that case. Also, considering these sandstorms only occur in a small number of specific places and are not always present, this shouldn't impact gameplay for you. This situation could be anolagous to having slowdowns when 25 or more NPCs are fighting for multiple sides simultaneously in front of you- one would expect the workload on the GPU and CPU to increase and the FPS to decrease.