If i wants to get best performance
 

and I wants to only pick one in the grafic setting. what will boost most to the performance?

But this is a question of aesthetics isnt it? By improving this or that option, that you think looks nice and is worth while.

I think a minimum of antialising is a must in games

Taking out some graphics effects might not give you a performance boost at all. Depends on whether you're CPU or GPU bound. SH is more CPU bound than your average game, so CPU is much more important that your video card.

Since shadows seem to have some problems on certain video cards, taking them out is the most likely option to give you a performance boost.