That option is only applicable for DX12 with multiple GPUs I think. You need to right click on rpcs3.exe and click on "run with high performance nvidia graphics" somewhere. This is true for every program that does not a have a driver profile, not just rpcs3.
Asus N55SF, i7-2670QM (~2,8 ghz under typical load), GeForce GT 555M (only OpenGL)