Did you check the hints from the Performance Optimization Guide in the help? It lists basically all that can be done to enhance performance.
In a nutshell: ideally both the Game FPS and the Direct Mode FPS should stay at 75 (DK2) at all times. If that is not possible, you should try to get the Game FPS to at least more than 40. vorpX uses asynchronous timewarp to create additional frames if you can’t reach 75, but you still need at least 40-50 game fps for a pleasent experience. The Direct Mode FPS absolutely have to be 75 for a judder free experience, no way around that.
You can display both values when you press [ALT][F]. What can be done to enhance performance is listed in the guide mentioned above.