If your TDS is zero it won't be the water.
It could be because the glass you are cleaning is fairly hot, this means the water will evaporate and dry out before any residual contaminants has been able to fully rinse away.
Spray the glass first to cool it down a little and spend a little more time rinsing.
If you have only just started using WFP then you also have to spend a lot more time cleaning the windows, you have to completely remove all old detergent residue from the glass and in particular, the edges where the glass meets the framework.
Any gaps, even the tiniest ones will gather dried in detergent residue, each time you use the WFP it the water will go into these gaps and water will bleed out of them, drawing out part of that residue each time. So you must watch out for that.
Is most of your work of the commercial variety? My sister lives out there and all the residential stuff has bars in front of the windows, my brother in law makes them!
If you are cleaning in the heat of the day then you are going to have to do a lot of rinsing.
But don't rely on holding the brush off the glass to do so, rest the tips of the bristles of your brush lightly on the glass then as you work your way down the pane of glass the bristles will also help ensure any particles are brought down the pane quicker.
Find a few accounts or windows that you can experiment on. The glass will dry out very quickly over there so it will only be a few minutes before you can check your results.
Oh, and don't forget to makes sure your brush is totally clean too!!
Regards,
Ian