First keep in mind that rendering stuff in stereo 3d, especially Geometry 3D, is a lot more demanding than rendering on your monitor, so if e.g. a game runs with 100FPS on your monitor, it will run at about half of that with G3D since everything has to be drawn twice.
That said:
Please check whether your headset is actually set to operate at 120FPS in the SteamVR settings. if that is the case, check the “Direct Mode FluidSync” option in the vorpX menu. Depending on the headset and the game the franmerate may be capped to make sure the GPU is harder to overload.
A word of caution: typically you don’t want to touch any of the headset sync options at all, usually the defaults are what is known to work best for a given headset. There is no harm in trying though. E.g. for some older games that you know for certain can reach 120FPS easily without causing the CPU/GPU to stall, it’s worth a shot to force FluidSync to ‘Off’ even for headsets where vorpX enables it per default.