I have the same issue on TP-Link Archer C2 v1 with OpenWrt 19.07.1. If I set "Fragmentation threshold" upload speed is maximum what is my internet but download speed slow down from ~200mbps to ~43mbps.
speedtest
[EDIT]
after few test I set all "Fragmentation threshold" and "RTS/CTS Threshold" to OFF. Then I set "Distance Optimization" to 1 ( what is true now, test router WDS is around 1meter from main) and i got download around 200mbps and upload at full speed of internet
speedtest-Distance Optimization
I'll check with local iperf3 servers
[EDIT ]
test iperf3 on rpi4 and omv server, rpi4 -> [LAN]tplink C2 ->[WDS 5Ghz] tplink archer A9 -> [LAN]switch ->[LAN] OMV server
pi@raspberrypi:~ $ iperf3 -c 192.168.1.5 -R
Connecting to host 192.168.1.5, port 5201
Reverse mode, remote host 192.168.1.5 is sending
[ 5] local 192.168.1.192 port 59304 connected to 192.168.1.5 port 5201
[ ID] Interval Transfer Bitrate
[ 5] 0.00-1.00 sec 25.0 MBytes 209 Mbits/sec
[ 5] 1.00-2.00 sec 26.6 MBytes 223 Mbits/sec
[ 5] 2.00-3.00 sec 26.5 MBytes 222 Mbits/sec
[ 5] 3.00-4.00 sec 25.5 MBytes 214 Mbits/sec
[ 5] 4.00-5.00 sec 26.2 MBytes 220 Mbits/sec
[ 5] 5.00-6.00 sec 27.3 MBytes 229 Mbits/sec
[ 5] 6.00-7.00 sec 26.2 MBytes 220 Mbits/sec
[ 5] 7.00-8.00 sec 26.7 MBytes 224 Mbits/sec
[ 5] 8.00-9.00 sec 26.4 MBytes 222 Mbits/sec
[ 5] 9.00-10.00 sec 26.3 MBytes 220 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.04 sec 264 MBytes 221 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 263 MBytes 220 Mbits/sec receiver
iperf Done.
pi@raspberrypi:~ $ iperf3 -c 192.168.1.5
Connecting to host 192.168.1.5, port 5201
[ 5] local 192.168.1.192 port 59308 connected to 192.168.1.5 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 20.4 MBytes 171 Mbits/sec 1 844 KBytes
[ 5] 1.00-2.00 sec 18.7 MBytes 157 Mbits/sec 0 720 KBytes
[ 5] 2.00-3.00 sec 17.2 MBytes 144 Mbits/sec 1 554 KBytes
[ 5] 3.00-4.00 sec 17.0 MBytes 143 Mbits/sec 0 595 KBytes
[ 5] 4.00-5.00 sec 18.9 MBytes 158 Mbits/sec 0 619 KBytes
[ 5] 5.00-6.00 sec 19.1 MBytes 160 Mbits/sec 0 634 KBytes
[ 5] 6.00-7.00 sec 19.8 MBytes 166 Mbits/sec 0 639 KBytes
[ 5] 7.00-8.00 sec 19.6 MBytes 164 Mbits/sec 0 653 KBytes
[ 5] 8.00-9.00 sec 18.6 MBytes 156 Mbits/sec 0 675 KBytes
[ 5] 9.00-10.00 sec 19.0 MBytes 160 Mbits/sec 0 696 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 188 MBytes 158 Mbits/sec 2 sender
[ 5] 0.00-10.02 sec 186 MBytes 156 Mbits/sec receiver
iperf Done.