This is know issue.
You can check a similar post on this forum http://forum.mikrotik.com/t/hap-ac-2-problems-extremely-poor-performance-found-in-2-4g-and-5g-wifi/118063/1
The Mikrotik engineers are working on a fix http://forum.mikrotik.com/t/v6-40-7-bugfix-is-released/118159/30