I'm sure an overly long string loop will weigh a few grains more than a 'proper' length loop and extra weight on the string will rob some arrow speed. You'd need a really good chronograph to see the difference though.

I often use a very long string loop to compensate for my overly long draw length. For a bow with a 32" draw, I need an extra inch of loop length to get to my anchor. I've noticed no ill effects from doing this. So, I'd say that having the loop the correct length for you to anchor properly is far more important than worrying about an extra grain or two of weight.