Twenty years ago when I was a kid working in a local archery shop, they sold "speed buttons", which were little rubber donuts, not much bigger than a brass nocking point. They claimed that adding them to the string, near the cam of compound bows, added speed. I never witnessed a comparison test and can't vouch for it...nor do I even wish to think about the dynamics of compound bows and decide whether that's feasible. But rest assured, any weight that you add to the center of a simple trad bow string will slow it down. Same thing as shooting a heavier arrow.
