I was thinking the same myself Squeaks! :
As for speed...
On small jobs like a 2 bed semi with perhaps half a dozen windows, the difference in speed is marginal, from stepping out of your vehicle to getting back in it to drive away there won't be that much difference.
But on larger jobs the difference can be staggering, and in particular on georgian and leaded work, even on a small job in those instances.
I've a couple of offices that used to take an average of 4 hours to do, three storey affairs and that would not include the frames either, add in the frames and that added another day to the job.
Now?
WFP takes under an hour and a half and that includes all of the frames, and I'm charging the same now as then...well, actually a bit more.
Your average casement window (3 panes of glass, 1 pane about 2' 6" wide by about 4' high, a narrow opening light about 3' long and 6" deep and one underneath about 3' long and 2' feet....figures may not be exact there, but you know the type I mean!!) will take an experienced window cleaner (trad) 90 seconds or so to clean.
WFP? Again, an experienced window cleaner will clean that same window in about 30 seconds, and that will include the frames too.
Of course, some trad guys will be able to do a window such as I described in not much more than 30 seconds....but that is virtually racing...no, hang on..it IS racing!
I have done countless time and motion studies, often on the window cleaner over the street who doesn't know I'm watching, and those figures bear out again and again.
But to be quick and do a good job takes practice, it's very easy to screw up with WFP...
Ian