Ap + sta+lan host internal congestion?

New to openwrt. I have two mt7621 board with openwrt 19.07 built in, the wifi chip is mt7612en. One board is configured as AP, the other is STA, and another non-openwrt board act as MASTER connected to AP with rj45 cable. The system is like this: STA sample pcm data at 44100sample rate,and send a 1200- byte udp pack per 14 ms to AP via wifi , then AP send the data to MASTER for playing back.But smthing not so good.
I grab udp packs by tcpdump on AP to find that at some time points, the wlan1 send more than one udp packs to MASTER simutaneously, making MASTER overrun.. Mostly the udp flow is right, ie the sending interval between two packs is 13-15ms, just like when they were sent from STA. So is that a congestion on the AP ?How to fix it ?
The mt7612e support 802.11ac, so maybe the wifi is fast enough? the following are used to set AP and STA respectively:
AP:
uci set network.lan.proto='dhcp'
uci set wireless.radio1.disabled='0'
uci set wireless.default_radio1.network='lan'
uci set wireless.default_radio1.mode='ap'
uci set wireless.default_radio1.ssid='mi6'
uci set wireless.default_radio1.encryption='none'
uci commit
ubus call network restart

STA:
uci set network.lan.ipaddr='192.168.18.18'
uci set network.lan.proto='static'
uci set network.wwan=interface
uci set network.wwan.proto='dhcp'
uci set wireless.radio1.disabled='0'
uci set wireless.default_radio1.network='wwan'
uci set wireless.default_radio1.mode='sta'
uci set wireless.default_radio1.ssid='mi6'
uci set wireless.default_radio1.encryption='none'
uci commit
ubus call network restart
Any suggestion is appreciated, pls help..