Help with TCP Retrans on LAN - Homehub 5A

I use 4 BT HomeHub 5a runing OpenWrt 23.05.3, 3 as access points and 1 as my router, they are connected via a Netgear S350 Series 8-Port Gigabit Ethernet Smart Managed Pro Switch.

If I test between the access points I consistently see slow data rates (50%) to device ending .251 and high restrans, where device.252 and .253 are consistency at a higher rate with no retrains. device ending .251 is lightly loaded with on 2 connected wireless devices during the test, where as .252 and .253 each have >10 connected Wifi devices during the test.

Any suggestion ?

 iperf3  -c 192.168.1.252 --bidir
Connecting to host 192.168.1.252, port 5201
[  5] local 192.168.1.251 port 43078 connected to 192.168.1.252 port 5201
[  7] local 192.168.1.251 port 43082 connected to 192.168.1.252 port 5201
[ ID][Role] Interval           Transfer     Bitrate         Retr  Cwnd
[  5][TX-C]   0.00-1.00   sec  12.6 MBytes   106 Mbits/sec    0    250 KBytes
[  7][RX-C]   0.00-1.00   sec  6.88 MBytes  57.4 Mbits/sec
[  5][TX-C]   1.00-2.00   sec  15.1 MBytes   127 Mbits/sec    0    341 KBytes
[  7][RX-C]   1.00-2.01   sec  5.38 MBytes  45.0 Mbits/sec
[  5][TX-C]   2.00-3.00   sec  15.1 MBytes   127 Mbits/sec    0    372 KBytes
[  7][RX-C]   2.01-3.00   sec  5.25 MBytes  44.3 Mbits/sec
[  5][TX-C]   3.00-4.00   sec  15.6 MBytes   131 Mbits/sec    0    372 KBytes
[  7][RX-C]   3.00-5.05   sec  9.50 MBytes  39.0 Mbits/sec
[  5][TX-C]   4.00-5.05   sec  0.00 Bytes  0.00 bits/sec    0    392 KBytes
[  7][RX-C]   5.05-5.05   sec  0.00 Bytes  0.00 bits/sec
[  5][TX-C]   5.05-6.00   sec  15.6 MBytes   137 Mbits/sec    0    413 KBytes
[  7][RX-C]   5.05-6.37   sec  6.38 MBytes  40.3 Mbits/sec
[  5][TX-C]   6.00-7.00   sec  10.1 MBytes  85.0 Mbits/sec    0    413 KBytes
[  7][RX-C]   6.37-7.60   sec  6.38 MBytes  43.7 Mbits/sec
[  5][TX-C]   7.00-8.00   sec  6.12 MBytes  51.4 Mbits/sec    0    413 KBytes
[  7][RX-C]   7.60-8.43   sec  4.50 MBytes  45.2 Mbits/sec
[  5][TX-C]   8.00-9.00   sec  8.62 MBytes  72.3 Mbits/sec    0    461 KBytes
[  7][RX-C]   8.43-9.09   sec  3.75 MBytes  48.2 Mbits/sec
[  5][TX-C]   9.00-10.01  sec  14.5 MBytes   121 Mbits/sec    0    461 KBytes
[  7][RX-C]   9.09-10.01  sec  4.75 MBytes  43.4 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID][Role] Interval           Transfer     Bitrate         Retr
[  5][TX-C]   0.00-10.01  sec   154 MBytes   129 Mbits/sec    0             sender
[  5][TX-C]   0.00-10.01  sec   154 MBytes   129 Mbits/sec                  receiver
[  7][RX-C]   0.00-10.01  sec  53.0 MBytes  **44.4** Mbits/sec  **441**             sender
[  7][RX-C]   0.00-10.01  sec  52.9 MBytes  44.3 Mbits/sec                  receiver
iperf3 -c 192.168.1.253 --bidir
Connecting to host 192.168.1.253, port 5201
[  5] local 192.168.1.252 port 46818 connected to 192.168.1.253 port 5201
[  7] local 192.168.1.252 port 46824 connected to 192.168.1.253 port 5201
[ ID][Role] Interval           Transfer     Bitrate         Retr  Cwnd
[  5][TX-C]   0.00-1.00   sec  11.5 MBytes  96.4 Mbits/sec    0    252 KBytes
[  7][RX-C]   0.00-1.22   sec  12.5 MBytes  85.7 Mbits/sec
[  5][TX-C]   1.00-2.00   sec  9.50 MBytes  79.7 Mbits/sec    0    325 KBytes
[  7][RX-C]   1.22-2.05   sec  9.38 MBytes  94.9 Mbits/sec
[  5][TX-C]   2.00-3.00   sec  9.38 MBytes  78.6 Mbits/sec    0    409 KBytes
[  7][RX-C]   2.05-3.00   sec  14.5 MBytes   128 Mbits/sec
[  5][TX-C]   3.00-4.00   sec  10.4 MBytes  87.0 Mbits/sec    0    447 KBytes
[  7][RX-C]   3.00-4.00   sec  15.8 MBytes   132 Mbits/sec
[  5][TX-C]   4.00-5.00   sec  10.0 MBytes  83.9 Mbits/sec    0    447 KBytes
[  7][RX-C]   4.00-5.07   sec  17.4 MBytes   137 Mbits/sec
[  5][TX-C]   5.00-6.00   sec  9.75 MBytes  81.8 Mbits/sec    0    447 KBytes
[  7][RX-C]   5.07-6.00   sec  14.2 MBytes   128 Mbits/sec
[  5][TX-C]   6.00-7.00   sec  11.8 MBytes  98.6 Mbits/sec    0    481 KBytes
[  7][RX-C]   6.00-7.02   sec  14.0 MBytes   115 Mbits/sec
[  5][TX-C]   7.00-8.00   sec  15.0 MBytes   126 Mbits/sec    0    481 KBytes
[  7][RX-C]   7.02-8.01   sec  7.25 MBytes  61.9 Mbits/sec
[  5][TX-C]   8.00-9.00   sec  12.8 MBytes   107 Mbits/sec    0    481 KBytes
[  7][RX-C]   8.01-9.00   sec  11.6 MBytes  97.9 Mbits/sec
[  5][TX-C]   9.00-10.01  sec  15.4 MBytes   128 Mbits/sec    0    481 KBytes
[  7][RX-C]   9.00-10.01  sec  14.8 MBytes   123 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID][Role] Interval           Transfer     Bitrate         Retr
[  5][TX-C]   0.00-10.01  sec   119 MBytes  99.9 Mbits/sec    **0**             sender
[  5][TX-C]   0.00-10.02  sec   119 MBytes  **99.8** Mbits/sec                  receiver
[  7][RX-C]   0.00-10.01  sec   132 MBytes   111 Mbits/sec    **0**             sender
[  7][RX-C]   0.00-10.02  sec   131 MBytes   **110** Mbits/sec                  receiver

fwiw, have you considered there may be a hardware fault ?

Try physically transposing two of the APs (eg. .251 and .252) to see if the issue stays with the .251 AP when used in other location?

I would also try replacing the .251 hardware and reload .251 config to new device to see if it makes a difference.

Thanks for the suggestion, got me thinking and I did something a little different I backed up the config of one of the "good devices" .252, took it offline, restored the .252 config to the .251 device and it cured the problem, so not hardware, or cable related but config related.

Suspect I tuned 252 and 253 in the past in some way and forgot to apply that to 251, I also have 254 which is my router and that has the same retrains issue, so I'll have a dig and see what I can find, might be useful for others if I cna find the config difference