Very slow tcp but fast udp

Hi!

There are two routers with openwrt and wireless connection between them. Routers are within less a meter distance.

If running iperf3 from one router to another I got strange results:

root@mirouter:~# iperf3  -c 192.168.6.1
Connecting to host 192.168.6.1, port 5201
[  5] local 192.168.6.181 port 40538 connected to 192.168.6.1 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec   199 KBytes  1.63 Mbits/sec   41   4.24 KBytes
[  5]   1.00-2.00   sec  1.04 MBytes  8.72 Mbits/sec   79   4.24 KBytes
....
[  5]   9.00-10.00  sec  1.61 MBytes  13.5 Mbits/sec  102   2.83 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  11.1 MBytes  9.33 Mbits/sec  1036             sender
[  5]   0.00-10.01  sec  11.1 MBytes  9.28 Mbits/sec                  receiver

UDP test:

root@mirouter:~# iperf3  -u -b 100M -c 192.168.6.1
Connecting to host 192.168.6.1, port 5201
[  5] local 192.168.6.181 port 56104 connected to 192.168.6.1 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec  11.9 MBytes  99.8 Mbits/sec  8612
....
[  5]   9.00-10.00  sec  11.9 MBytes  99.9 Mbits/sec  8628
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec   119 MBytes   100 Mbits/sec  0.000 ms  0/86314 (0%)  sender
[  5]   0.00-10.00  sec   119 MBytes  99.4 Mbits/sec  0.122 ms  494/86314 (0.57%)  receiver

but the most wired thing is if running 100 thread tcp test:

iperf3 -P 100 -c 192.168.6.1
....
[SUM]   0.00-10.00  sec   159 MBytes   133 Mbits/sec  30385             sender
[SUM]   0.00-10.02  sec   155 MBytes   129 Mbits/sec                  receiver

if to connect routers from the PC the connection speed is normal

openwrt is from developer snapshot

kernel is:

dirouter ~ # uname -a
Linux dirouter 5.4.51 #0 SMP Sun Jul 12 08:17:28 2020 x86_64 GNU/Linux
root@mirouter:~# uname -a
Linux mirouter 5.4.52 #0 SMP Tue Jul 21 20:44:37 2020 mips GNU/Linux

wireless

MediaTek MT76x2E 802.11acn
and
Qualcomm Atheros QCA9880 802.11acbgn

both in 5Gz mode connected in Client (WDS) and Mater (WDS) respectively

normal tcp are limited

could you give more details please