What does the AUTO DROP-DOWN setting in the OFP Preferences program do?
AUTO DROP-DOWN determines to what extent OFP should dynamically reduce the texture details during gameplay. A setting of NONE means OFP will not reduce texture detail, resulting in higher quality visuals at the expense of requiring more processor power. Those on lower performing systems may wish to try setting AUTO DROP-DOWN to 4x or 2x to improve performance.
More information on tweaking OFP can be obtained through this FAQ item.