Does decreasing the length of the line between a harness and zip wire increase the velocity at which you travel? My friend is an instructor at an outdoor adventure centre which has lots of zip wires. The instructor's harness has a shorter line that attaches to the zip wire than the normal harness does and she claims that you go much faster with it. Is this true? I can't think of any reason as to why it would be MUCH faster but apparently their speed gun proved it.
Edit: Diagram for clarity

 A: 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.
A: I know this question is years old, so the OP has probably moved on, but for the sake of those stumbling on this years later....
The quick answer is no, the length of the lanyard/tether, doesn't have any noticeable effect on the speed of the rider. What it may affect though, is rider positioning, which has a significant effect on wind drag and will change the speed.
For the last 15 years I have been a guide and then a designer/builder of such zip line tours. There are four primary variables that factor into the forces on a rider and their speed: rolling resistance (decelerating force), wind resistance (decelerating force), rider weight (accelerating force), and cable slope (accelerating force). Lanyard length technically would increase cross-sectional area which would increase wind resistance, but this would be negligible compared to other variables.
Rider weight and rolling resistance tend to be pretty static. Cable slope changes along the length of the cable, depending on installation pitch and tension (it's steeper at the beginning and flattens out towards the end), but also the amount a rider deflects the cable (a heavy person will deflect the cable more and therefore their RIDE PATH will be steeper at the beginning than a light persons ride path). Wind resistance is the hardest to account for because it depends on so many things like the direction and speed of the wind that day, the size of the rider, the clothes the rider is wearing, and most importantly, the rider positioning.
As a guide, we would always tell participants about "starfish" vs "canon ball" position. These drastically change the cross sectional area of the person, and therefore the amount of wind resistance. The very fastest ride position is "the pencil" which has the least wind resistance.
I experimented as a guide and could ride the same zip line on the same day with the same weather conditions, and the same hardware setup (same gear weight and rolling resistance), but ride one time in the starfish position and not even make it to the end, but the next time in the pencil position, and approach the braking zone at 40+ km/h.
So back to the question....
The lanyard length would have zero effect on rider weight, rolling resistance, or cable slope, but could effect the rider positioning which in turn would affect wind resistance.
The shorter guide lanyards are designed so guides can control their braking with a gloved leather hand on the cable. My theory is that the guide in question naturally leans back to avoid rubbing her helmet on the cable, which puts her in a more pencil-like form. The participants on the other hand, are having a grand old dangle on the longer lanyard and flapping in the breeze. So their wind resistances are very different.
