Are you asking about a string loop or the metal loop? The metal loops are absolute junk. They slow the bow down about 10fps, can actually break your string under the serving, make more noise, beat the he!! out of your cables. And if the screws come loose you can possibly have a real nasty accident, from a good punch in the mouth to losing an eye.

String loops are lighter and do virtually no damage to the string, last a long time, and don't cost near as much. Speed loss is about the same as one brass nockset. I've actually chrono'd my bows with and without the brass nocksets and the speed loss with them is about 4fps per ncokset.

Hope this helps.