The reason it takes more effort to keep a rope tight is because of the stretch in the rope and the effects of gravity on the extra weight of the rope. As far as it affecting the belt and what would be referred to as belt lash, it would be negligible because the long section is before the SC pulley, not after it. Think of a belt-driven motorcycle and how one side is tight and the other isn't. Same thing here only with a lot more pulleys.

Belt.jpg