Debugging transmission rate

I have an issue with tx bitrate dropping to 650, 780 and back despite perfect (-40 db) signal and no other APs around. Router is redmi ac2100.
When tx rate does not drop iperf test shows 570 mbit which is close to stock firmware.
However when tx rate starts dropping it goes from 350 to 500 mbit. It happens with all devices I have.

Rate stays steadily at 866 for few minutes after restarting wireless and then it starts dropping. After a while it gets better, but starts dropping again randomly.