Why is it that a Newton's Cradle on a slope comes to a stop faster than on a level surface?

Interesting question. Think about how the direction of impact between balls causes a net force with components parallel to and perpendicular to the direction of motion.

I guess it's partly to do with having more exposure to air resistance at impact. But I feel it has something to do with gravity making it harder for the kinetic energy to travel in a more upwards direction. Is that along the right track?