It may be worth try disabling flow control as that should help improve performance further. If you wish to try something that may help:
ssdk_sh flow status set 0
ssdk_sh port flowCtrl set 1 disable
ssdk_sh port flowCtrl set 2 disable
ssdk_sh port flowCtrl set 3 disable
ssdk_sh port flowCtrl set 4 disable
ssdk_sh port flowCtrl set 5 disable
ssdk_sh port flowCtrl set 6 disable
This will need to be done via the shell. You can use the web terminal — just shift+click the device name from the Network tab and paste the commands there. There’s a way to make the change permanent too, but I’d suggest testing it first. I’m not sure what the expectations are. I don’t expect it to completely fix the issue, but I think it’ll probably improve things a bit.
That would disable it on all ports. The port mapping is as follows:
WAN1: ssdk port 4
LAN1: ssdk port 3
LAN2: ssdk port 2
LAN3: ssdk port 1
LAN4: ssdk port 6
WAN2: ssdk port 5
EDIT: if you wanted to try the physical WAN2 port only, then it would just be:
ssdk_sh flow status set 0
ssdk_sh port flowCtrl set 5 disable