The limiting factor on a first stage is often the size of the orifice leading into the regulator from the tank.
Balanced first stages are able to have a larger orifice without encountering problems with excessive IP drop as the tank pressure goes from full to empty caused by the change in downstream assist provided in the unbalanced system.
At the same time a balanced first stage can have a larger orifice and can then also benefit as well from larger internal passages that tend to promote increased flow rates at higher absolute pressures.
Consequently, unbalanced first stages tend to have lower flow rates, especially at low tank pressures as you can only get x amount of air thorugh a given orifice size at a given pressure, so both a smaller orifice size and lower supply pressures will reduce the potential flow rate of the first stage.
This lack of flow at the first stage begins to effect the performance at the second stage when the sustained flow required of the first stage is such that the Intermediate pressure drops to the point that additional suction is required to open the second stage poppet and keep the gas flowing.
Deeper depths are more demanding as the volume of gas required to supply the diver at, for example, 198 ft is seven times greater than at the surface. At the same time, the actual pressure of the gas flowing through the reg is going to be 88 psi higher at 198' than at the surface due to the increased ambient pressure acting on the first stage and this increase in denisity creates a slight increase in viscosity with will magnify any flaws or obstruction in the gas passagwasy in the regulator.
So it's a challenge for a reg to supply the neccesarily large volume of gas at depth with a low inlet pressure and if it does well there, everything else is gravy.
That said WOB numbers are very misleading and can be manipulated to become even more misleading. They do not speak to the quality of the gas flow and they do not fully address other more subjective and often more important factors such as cracking effort. For example a second stage that requires a higher than desireable cracking effort (the effort needed to initiate flow) but them literally inflates you with positive pressure may average out well on a WOB chart, but not feel particularly good to breathe off off compared to a smoother and more naturally breathing regulator with a lower cracking effort but with a higher overall WOB. Personally, I totally ignore WOB numbers unless they are given with comparisions at different depths and flow rates where the differences can then be very illuminating.
