I recently bought 10 lpm pump, had a few problems setting it up on controller so am running it direct from battery for now, the flow rate is brilliant, you do use a lot more water but working time is a lot quicker, I will sort controller out and prob run at 7-8 lpm,
These windows in question, look worst at the bottom of the glass, if so it could be stained off water limescale dropping onto the sill and splashing back, over long periods it builds up.
I took over a job that had a broken gutter with windows below, the glass had a build up of white stuff down the middle with lots of splash marks on the glass, felt rough to touch, trad cleaned the window but it kept coming back, tried scraping it off but the same, tried a few chemicals and in the end put a bottle of vinegar in a small bucket and spent an hour just scrubbing with microfibre, left a few small marks but done the job, I was surprised