Does automated backup performance suffer if backup completion time exceeds the backup window?

Is there anything "special" about the Amazon RDS Automated Backup Window, or is the backup window merely an arbitrary timespan in which the backup is expected to complete?

That is to say, if I have a backup window of 30 minutes, and the backup exceeds 30 minutes... will there be a noticeable performance difference in the progress/completion of the backup during the timespan after the backup window? Would increasing the backup window help in that situation? I have a suspicion that additional resources may be allocated for the backup procedure during this window, but I haven't yet found anything in the docs to suggest that this is or isn't the case.

If the behavior diverges across database engines, I'm specifically interested automated backups for MySQL 5.6.xx with InnoDB.