Can someone help me with optimizing a script?

I have made a gun and it works really well...if you're on a server the arms lag I know its because of how I'm positioning them I'm removing the Welds and then cframing them to the player yes this is in a OnRenderStep but I know if I make new welds then manipulated the welds the arms would not lag the problem is manipulating welds is confusing can someone explain how the work :/