The graph of energy stored is completely wrong. Even if the limbs are pre-tensioned (actually they are pre-tensioned on standard self bow as well, just a bit smaller amount) the bowyer always starts the pull at zero and from there the force curve rises to full draw. The tension in the limbs and also the rigid ends that act as levers cause the curve be more steep at the beginning of pull and increase the surface which represents energy stored even with similar final draw weight. But in the end efficiency of the bow is what matters and bows with similar or same "curve" can in the end shoot the same arrow at lower speed. Efficiency of shooting s also dependent on arrow and string weight...
Modern compound bows are engineered for specific shape of this curve that lifts up in the middle to top draw weight and sinks down at the end so you don't have to hold full draw weight while taking aim.
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