A customer had to restrict the balancer window to only 4 hours per day, when traffic is much lighter.
Chunk migrations keep falling behind due to the small balancing window.
The fix was to enable longer hours over the weekend, which also shows a much lighter traffic for that period.

Since the balancer schedule only supports hours, alternatives were:

• manually change the schedule on Friday night and Sunday night
• have a cron to make the changes to the schedule.

In this case, it would have been helpful to be able to specify the hours per day in the schedule.