The muzzy rest will not decrease your fps , if anything it will increase .
I have used them since they came out with the first model , fist year production with the roller cabel gaurds . They are a good rest the only drawback I have had with them is the arm cracking , and the hook twisting . If you catch the hook on something , say when doing a stalk hunt it has a tendancy to twist effecting height ,something I have found that helps is to cut some of the hook off so it does not stick out so far and catch things . You would think tightning down the allen screw that holds the arm in place would help , but thats how they crack , nothings perfect ,the muzzy has been a good rest in its time , but the limbdriver is taking its place on my bows .