I think this is why most bow companies give you an IBO speed range now. My Slayer was rated at 350 f.p.s. but at 72 lbs, 31" draw and 380 grain arrow, I can get best speed of 312 f.p.s. I know arrow is 30 grns over IBO but I am pulling 1" longer draw which should be a slight off set. Also 2 extra lbs of weight.