If you are using a trolley or a backpack, or have limited water carrying capacity then how much water you are using is vital, so getting it just right is an artform in itself!
I've found that a flowrate of between 2l and 2.5l per min is my ideal.
But using more water is also a skill too, well, to get the most out of it anyway, faster the flowrate the greater the potential to work faster too, at least on repeat cleans.
you have the flowrate up to high and you are wasting water, but you have to be able to work at a speed you can cope with too.
Matt has said that he experimented with a higher rate and earned no more than he did at his normal rate, but it takes time to get the speed you are actually working at to change, if you spend the same amount of time as before on a window then all you are doing is using more water
We all develop our own techniques to cope with whatever flowrate we feel comfortable with.
As can be seen by all the varied replies, we all use different flowrates!!!
Ian