I do think the Issue is somewhere else.

My setup from office (lab) to testserver:
(Kali Linux 2022.3) Lenovo X200s - (1m LAN) - Beryl [GL-MT1300] - (30cm LAN) - Netgear GS808E - (7m LAN) - Wall outlet - (25m LAN) - Patch panel - (1m LAN) - Netgear 16 Port Switch - (1,5m LAN) - Proxmox Cluster - Debian 11.4 as container.

I do think this is a very casual setup, not very optimized.

Result:

root@penelope:~# iperf3 -s
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------
Accepted connection from 192.168.21.161, port 48496
[  5] local 192.168.21.15 port 5201 connected to 192.168.21.161 port 48512
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   110 MBytes   924 Mbits/sec                  
[  5]   1.00-2.00   sec   112 MBytes   936 Mbits/sec                  
[  5]   2.00-3.00   sec   111 MBytes   931 Mbits/sec                  
[  5]   3.00-4.00   sec   111 MBytes   934 Mbits/sec                  
[  5]   4.00-5.00   sec   111 MBytes   932 Mbits/sec                  
[  5]   5.00-6.00   sec   111 MBytes   934 Mbits/sec                  
[  5]   6.00-7.00   sec   111 MBytes   935 Mbits/sec                  
[  5]   7.00-8.00   sec   111 MBytes   933 Mbits/sec                  
[  5]   8.00-9.00   sec   111 MBytes   929 Mbits/sec                  
[  5]   9.00-10.00  sec   111 MBytes   930 Mbits/sec                  
[  5]  10.00-10.00  sec   510 KBytes   894 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.00  sec  1.09 GBytes   932 Mbits/sec                  receiver

The path:

lupus@kira:~$ traceroute 192.168.21.15
traceroute to 192.168.21.15 (192.168.21.15), 30 hops max, 60 byte packets
 1  console.gl-inet.com (192.168.8.1)  0.651 ms  0.789 ms  0.859 ms
 2  192.168.21.15 (192.168.21.15)  1.981 ms  1.995 ms  2.217 ms
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