Dual display cause performance drop down

I found if my computer connect 2 displays and set display mode as DualView in control panal,the OGL rendering performance of my program is dropped down greatly,this is why?Is there any solution to solve this problem?
My computer use GF8800GTS card,with 175.19 driver.

Adjust settings in driver control panel. Search for:
HW acceleration
and change option to one of:
Single display
Multi display performance
Multi display compatibility

according to your needs.

and anyway for highest performance, use “span” mode instead of dualview.