We have released 6.43rc RouterOS version that includes potential fix for wireless performance problems on RouterBOARD hAP ac^2 and cAP ac.
Please upgrade and test. Report results back to support@mikrotik.com if you still experience problems even after an upgrade.
“wireless - improved wireless throughput on hAP ac^2 and cAP ac”
http://forum.mikrotik.com/t/v6-43rc-release-candidate-is-released/118681/52