Different VideoRays produce different amounts of thrust. We'll answer this question for the model with the most thrust, the Pro 4.
Tether pulling ability is a function of the amount of thrust the vehicle produces, which is countered by both the weight of the vehicle, and the drag from moving through the water. If there is a current, and the vehicle needs to turn into it, this can be another limiting factor - see currents FAQ.
VideoRay ROVs have by far the best power to weight ratio available in vehicles in this weight class with up to 1.7 times as much thrust as it weighs (18 pounds of thrust on a 10.6 pound vehicle.) In addition, the hydrodynamic shape of the VideoRay means that more of that power goes into dragging tether rather than overcoming hydrodynamic drag.
Keep in mind that long tethers rob the vehicle of power.VideoRay's modular tether allows you to optimize your lengths and types of tether used on each mission. See FAQ about tether types.