If CG is forward, the nose is heavier, and the horizon stabiliser has to generate more down force (through up elevator) to balance. This larger down force has to be countered with more lift, and higher speed.
A rearward CG requires less tailplane downforce, and the stall speed reduces. There is also less drag generated.
Va is normally considered as maximum speed that any one control can be fully deflected without overstress (and even then, I believe, just the one time. Not full left to full right rudder…). Va reduces as weight decreases.