I thought the same colin...iv never used anything else but that stuff took ages! It was a chippy i used it on and it had two large windows that were stained like mad, 700quid to replace them both. The owner said to me that had had about 15 window cleaner out to it and no one could touch it. All the paint off the metal frames started to flake away because that had tried turps to get it off 
. Danny to the rescue.....she was over the moon with the end result. I wasn't sure if it was me doing something wrong and that's why it took so long. Where did you try it colin?
Danny
Mine was used on some leaded windows on a pub
But there was a hanging basket with a snapped
Chain above 2 windows and the rain had emptied
Soil all over them and intern left terrible deposits
All over the glass, I got rid of the stains but I felt there
Must be something better than this!
It was block lead so I had to do each individual pain
Took ages
Lesson learnt mate