On OA 7.x Update named or Update PowerDNS tasks execution time gradually increases with time. The effect is more visible, when many DNS zones are synchronized at the same time.
/var/log/pa/core.log shows a large amount of such entries at the time of the task execution:
Dec 11 13:25:36.267 : DBG [task:58394544:744530 RequestProcessor-162838 pau 742366617]: c.p.p.tracer entry: com.parallels.pa.service.notifications.ejb.OnScreenNotificationManagerBean.getLastMessagesByContext(null, UPDATE_DNS_ZONE:197881*, inProgress) Dec 11 13:25:36.267 : DBG [task:58394544:744530 RequestProcessor-162838 pau 742366617]: c.p.p.s.a.e.UserManagerBean locating user by UUID null Dec 11 13:25:36.269 : DBG [task:58394544:744530 RequestProcessor-162838 pau 742366617]: c.p.p.s.a.e.UserManagerBean user with UUID null not found Dec 11 13:25:37.432 : DBG [task:58394543:744509 RequestProcessor-162839 pau 742363379]: c.p.p.tracer exit: com.parallels.pa.service.notifications.ejb.OnScreenNotificationManagerBean.getLastMessagesByContext returning 
Since OA 7.0, the system keeps records for CCP v2 on-screen notifications that are checked on each DNS update task. The amount of records grows with time and does not get rotated, decreasing the speed of DNS synchronization.
The issue is planned to be improved in scope of requests PFR-1223 and PFR-1224.
Contact Odin Technical Support to optimize the system performance.