Why does unifi ap ac lite transfer speed drop?

Question

WiFi

Good evening.
there is a unifi ap ac lite point, it has 3 networks: 2 at 2.4GHz and 1 at 5GHz, all are called differently.
Phone (galaxy s6) connects to 5GHz network 5GHz
network:
Channel 42(36,+1) VHT80
Transmit Power 20 dBm (EIRP)
WPA Personal AES/CCMP only
No standard groups used (wlan, user)
Controller on ubuntu16.04
Guest networks are not used by means of the unifi controller, only 1 network from 2.4 in a separate vlan.
Uplink Connectivity Monitor is disabled.
Problem:
After connecting, the transfer rate is normal (300+ Mbps in both directions), iperf2 is measured, after a while the transfer rate drops to 30. It is solved by reconnecting to the network (reconnect via unifi controller also helps) the air is clean
Where to dig?
The problem is not with the phone. Before that, wifi was distributed by a regular pci card via hostapd and there were no such problems (although wifi was n with a channel width of 40 MHz)