FluidSync indeed mainly caps the game framerate at half the headset’s refresh rate. It’s a bit of a matter of taste, but personally I prefer that over fluctuating framerates when 90fps can’t be reached reliably. The default (‘Auto’) switches it on/off depending on how close the game is to potentially reach 90fps, but you can safely turn it off entirely if you prefer that.
Async is a bit difficult to explain in a non-techie way, so bear with me: you have to imagine vorpX as two rendering processes in parallel: the game and as a second one the rendering to the headset. The async option defines wehther both are in-sync with each other or not. You should usually leave the async optin on, that allows the render to headset thread to run faster than the game.
Performance wise there are always some experiments but I can’t really share any details on ongoing development. In any case you shouldn’t expect wonders like magically doubled G3D framerates.