If you use Windows 10 Anniversary: the current Direct Head Tracking implementation does not work correctly with Windows 10 Anniversary, resulting in stutter. The next vorpX will do this in a different way that also works on Win 10 Anniversary.
If the Direct Mode FPS is constantly below 90fps, you need to reduce the load on your GPU for example by reducing game details, resolution, switching to Z3D. Also please leave FluidSync enabled if disabling it makes things worse. If it is enabled per default on your headset, that is done for a reason.