Speed loss because of the biscuit on a tuned bow with 4" straight vanes will be on the order of 1% of the FPS of the same bow with a dropaway. If the biscuit is flattened on one side to reduce its thickness by half the speed loss can also be cut in half. If you shoot 5" helical you may lose 2% of the arrow FPS. If you use 3" feathers in a straight fletch the actual loss in FPS can be real hard to measure. Happy holidays and Good luck hunting!