If these windows take that much longer to clean, let someone else clean them and go off and get an easier customer. The beauty of this malarky is working where, when and for who you want. Why do a job that takes 30 minutes for example and get 15 quid, if you could do 2 smaller jobs at 10 quid each in the same time?
If the price don't fit the time it takes, move on. Only my opinion, but I hate filddly crap windows like that, so I don't have anyone on my books with those.
Good luck with whatever you decide though.