If you are experiencing slow network performance on a Linux server, it might be due to offloading features of the network card. Offloading is a technique used by network cards to offload the processing of network traffic from the CPU to the network card. This can improve performance by reducing the load on the CPU, but it can also cause problems if the network card is not properly configured.

To check the current offloading settings of a network card, you can use:

ethool -k eth0

To disable offloading features, you can use:

ethtool -K eth1 tso off gso off gro off

To enable them again, you can use:

ethtool -K eth1 tso on gso on gro on

For example, this is the output of iperf3 with offloading enabled:

-----------------------------------------------------------
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  83.4 MBytes   700 Mbits/sec
[  5]   1.00-2.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   2.00-3.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   3.00-4.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   4.00-5.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   5.00-6.00   sec  82.6 MBytes   693 Mbits/sec
[  5]   6.00-7.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   7.00-8.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   8.00-9.00   sec  82.5 MBytes   692 Mbits/sec
[  5]   9.00-10.00  sec  82.4 MBytes   691 Mbits/sec
[  5]  10.00-10.00  sec   191 KBytes   691 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.00  sec   826 MBytes   693 Mbits/sec                  receiver
-----------------------------------------------------------
Server listening on 5201 (test #3)

And after disabling offloading:

[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec   113 MBytes   948 Mbits/sec    0    379 KBytes
[  5]   1.00-2.00   sec   113 MBytes   945 Mbits/sec    0    379 KBytes
[  5]   2.00-3.00   sec   112 MBytes   942 Mbits/sec    0    397 KBytes
[  5]   3.00-4.00   sec   112 MBytes   942 Mbits/sec    0    414 KBytes
[  5]   4.00-5.00   sec   112 MBytes   939 Mbits/sec    0    414 KBytes
[  5]   5.00-6.00   sec   112 MBytes   941 Mbits/sec    0    414 KBytes
[  5]   6.00-7.00   sec   112 MBytes   943 Mbits/sec    0    414 KBytes
[  5]   7.00-8.00   sec   112 MBytes   941 Mbits/sec    0    414 KBytes
[  5]   8.00-9.00   sec   112 MBytes   942 Mbits/sec    0    414 KBytes
[  5]   9.00-10.00  sec   112 MBytes   939 Mbits/sec    0    414 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  1.10 GBytes   942 Mbits/sec    0             sender
[  5]   0.00-10.00  sec  1.10 GBytes   941 Mbits/sec                  receiver

if you disable offloading you may notice more CPU usage, but the network performance should be better.