With decent hardware (less than 6 years old so to speak :p), performance loss is inexistant, wichever scale the battle is ; hitting constant 60fps in this old game is really not a challenge for current PCs, especially on mid/high-end hardware.
All performances hits in RoR are more likely due to computing side (CPU and server) more than the graphical side.
But by experience, in recent 3D-demanding games, you may lose about 5-10 fps max, using a fair amount of reshade techniques, like faked AAO wich is the most heavy one.