I have used Beaver Balls on my last three Mathews bows. They are deadly quiet, durable and also act as wind checkers. You shouldn' t need to replace them unless you shoot the same bow for many years. Far as speed loss goes, I would imagine that they are similar to any other string silencer (3-4 fps loss). I have tried just about every other string silencer on the market and I ALWAYS come back to my Balls after a few days.