I think a lot to do with time is confidence to know when a window is clean and move on to the next one. Obviously lots of other factors come into it as well, brush flow rate etc, etc, but nearly every wfp window cleaner I have seen working uses more water than needed and spends much more time than needed trying to clean a clean window. I have advised people in the past to clean a window that they are going to be doing the inside of as normal and do the one next to it in half the time, when inside examine them closely and see if there is a difference. Like I say other factors come into play and like was mentioned previously you should know exactly what a window needs before you even touch it. There has been many discussions over the years and in many different places about what make a good professional window cleaner and that is one of them. You have to adapt quickly to the environment your working in, just because a house is done in 5 minutes doesn't mean every house is done in 5 minutes. If one is really dirty as maybe it's a 2 monthly, or there's been a storm, it may take 6 minutes and a little extra water. Its all about knowing what's needed before you even start, and having the confidence to move on when the window has had enough. To stand at a window for a set amount of time to make sure you cover every eventuality is ludicrous, and will be costing you vast amounts of time and money in the year.