Hi, 
I'm testing the new Rev.K board and facing troubles with uplink performance. Downlink is ok. I already asked on Olymex forum but no response so far. 
$ iperf3 -c 192.168.0.110
Connecting to host 192.168.0.110, port 5201
[  4] local 192.168.0.104 port 45700 connected to 192.168.0.110 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec  3.59 MBytes  30.1 Mbits/sec  708   4.24 KBytes       
[  4]   1.00-2.00   sec  4.14 MBytes  34.8 Mbits/sec  964   2.83 KBytes       
[  4]   2.00-3.00   sec  4.19 MBytes  35.1 Mbits/sec  956   2.83 KBytes       
[  4]   3.00-4.00   sec  4.16 MBytes  34.9 Mbits/sec  944   2.83 KBytes       
[  4]   4.00-5.00   sec  4.02 MBytes  33.7 Mbits/sec  925   5.66 KBytes       
[  4]   5.00-6.00   sec  3.03 MBytes  25.4 Mbits/sec  605   2.83 KBytes       
[  4]   6.00-7.00   sec  4.02 MBytes  33.7 Mbits/sec  904   2.83 KBytes       
[  4]   7.00-8.00   sec  3.87 MBytes  32.4 Mbits/sec  859   2.83 KBytes       
[  4]   8.00-9.00   sec  4.08 MBytes  34.2 Mbits/sec  928   2.83 KBytes       
[  4]   9.00-10.00  sec  4.14 MBytes  34.7 Mbits/sec  955   2.83 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  39.2 MBytes  32.9 Mbits/sec  8748             sender
[  4]   0.00-10.00  sec  39.2 MBytes  32.9 Mbits/sec                  receiver

Downlink:

$ iperf3 -c 192.168.0.110 -R
Connecting to host 192.168.0.110, port 5201
Reverse mode, remote host 192.168.0.110 is sending
[  4] local 192.168.0.104 port 45704 connected to 192.168.0.110 port 5201
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-1.00   sec   106 MBytes   890 Mbits/sec                  
[  4]   1.00-2.00   sec   109 MBytes   915 Mbits/sec                  
[  4]   2.00-3.00   sec   111 MBytes   934 Mbits/sec                  
[  4]   3.00-4.00   sec   111 MBytes   935 Mbits/sec                  
[  4]   4.00-5.00   sec   111 MBytes   934 Mbits/sec                  
[  4]   5.00-6.00   sec   111 MBytes   935 Mbits/sec                  
[  4]   6.00-7.00   sec   111 MBytes   934 Mbits/sec                  
[  4]   7.00-8.00   sec   111 MBytes   934 Mbits/sec                  
[  4]   8.00-9.00   sec   111 MBytes   935 Mbits/sec                  
[  4]   9.00-10.00  sec   111 MBytes   934 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  1.08 GBytes   931 Mbits/sec   37             sender
[  4]   0.00-10.00  sec  1.08 GBytes   928 Mbits/sec            

The same behavior is repeatable with the official Olimex firmware release #115,  with Armbian Stretch and with Buildroot (fixed u-boot 2018.09, CONFIG_GMAC_TX_DELAY=4).  Did anyone face the same issue?

I found out what was wrong. The TP-Link router in between works the strange way by limiting the download. Direct connection to PC gives 700 Mbit/s uplink which is quite good and maybe even tweakable with CONFIG_GMAC_TX_DELAY.