I did few more tests.
First I tried to plug in pc directly to the router (without all the sockets). Exactly same result.
Then I tried to remove master-port value from all Eth2-Eth4 ports and added them to Bridge. And the issue gone. The speed is normal 80/80. Tried with both connection (direct and through the sockets). What does it mean? Is there some defect with hardware switch?
Maybe this is somehow related to this topic:
http://forum.mikrotik.com/t/hap-ac-faulty-seriers-very-poor-lan-performance-switch-problem/96655/1