David,
To begin with you will be slow with WFP, you will also leave spots...its the learning curve..it goes with the territory I'm afraid
If you want an idea as to how long it should take you to clean..say, a normal size 3 bed semi, with perhaps 10 or 12 windows, well it should take you somewhere around 10 minutes from start to finish (pulling up to driving away)
You'll need a fairly high flow rate, but it also takes quite a bit of experience to work efficiently with a high flow rate, you have to work fast or you'll be wasting water.
Concentrate on the top edges.
for instance; I have always washed the frames as well (wherever and whenever possible) so for me the top edge includes the top of the frame.
If the windows are dirty then the top edge of the frame is too (even if it looks really clean) so I always wash the top from side to side quite quickly but also thoroughly, then kind of wedge the bristles of the brush up into the top of the pane of glass where it meets the frame and once more it is quickly scrubbed from side to side before working down onto the glass.
I don't waste time rinsing on the glass, put rather ease up on the pressure on the brush so that the bristles are just resting against the glass.
It takes practice and confidance to get fast, but work at it and you'll get there.
It will take you an enormous amount of practice to become proficient with a pole and squeegee on upstairs windows too, to do it properly takes a high degree of skill.
And so far I've yet to find a decent scrim clamp! Well, ok, the only scrim clamp I've used is the Unger one, and it is pants.
An expensive piece of plastic...crap!
As you are already WFP, and you also have no intention of climbing the ladder to clean windows, as has already been said, WFP the tops and trad the bottoms.
If the windows aren't really suitable for WFP, tell the customer that they'll have to put up with the tops not coming up to scratch (you sure as hell won't do a better job with a pole and squeegee anyway) and tell the customer that you will do the lower windows the traditional way.
Ian