just see this topic http://forum.mikrotik.com/t/xiaomi-phone-low-wifi-tx-rate/93622/8
mikrotik confirm this issue but ignore it, and with high probability they ignore and your issue to…
I have Xiaomi redmi note 2 (my frends have different phones Xiaomi and they have the same issue), and the same like you — not higher than 20 mb/s.