Leeches generally put a little weight on the string. My chrono says that put approx. half way between the can and the center serving they reduce speed by 2-3 fps. If your end serving isn't too long you can run them closer to the cam (cams in my case). Moving them closer reduces the speed loss and can even make them act like speed balls, reducing some string oscillation and gaining a couple fps. It all depends on where they are located on the string. If it's for a hunting setup I always move them a little toward the center to get the best noise reduction/speed combo.