My first guess is that if feels faster since your eyes are closer to the line zipping by. This the same effect that makes it look like you are going slower down the highway compared to distant buildings than to stuff at the edge of the road.
The shorter line would have slightly less wind resistance, but that should be a insignificant effect in the scheme of things.
Someone mentioned the angle of the dangle would be different between someone hanging from a long teather and a short teather, but there should be no difference for the same person as long as the weight of the teather is small compared to the weight of the body.
The weight of the person could matter though. One clue is you said "she" for the instructor. Women, especially physically fit women likely to be such instructors, will be lighter on average than the average customer. She could indeed be zipping faster than most others, although this has nothing to do with teather length. One flaw in your diagram is that you don't show the zip line sagging from the weight of the person. The more the sag, the less of a downhill push the person gets. Think of the limiting case where the line sags so much that you stop at a dead spot before the other end where you are lower than both ends.