Paying a money bonus reward for working faster and achieving more during the day is a road to trouble IMHO. The motor trade is plagued with the aftermath of their bonus insentive schemes they introduced into their workshops many years ago.
All the mechanics think about is how much bonus they will earn at the end of the week. Quality of workmanship drops as they 'cut corners' to do the job quicker. They don't want to trouble shoot a problem as they will end up loosing bonus if they spend time on trying to find an intermittant problem and don't suceed. If they are told to do it they later report back that they couldn't find the problem when they didn't even look.
We had one character who was used as an example of achievement and dedication through our motor group due to his hard work. He was later caught out charging and completing 2 services on 2 cars that he never touched. But he still managed to fill in the customer's service book.
Unfortunately, this sort of cut corners approach is also a reality with window cleaning. This also will reflect on your reputation as a business and if you are small business, directly on you. Once you start employing you have to be proactive by taking time out to randomly inspect the quality of their work and sort out any issues before the customer complains. (This I learnt from what was done and not done in our service workshops. Our workshop manager should have been walking around his workshop most of the day keeping an eye on what each of them were doing. He didn't - hence the drama. )