There is not a lot of information out there about viewing the network interface throughput in real-time via CLI on Nexenta out there on the internet, so I hope this helps someone.
I discovered you can view the throughput in real-time by using a flow control monitor, and flowadm.
First, enter expert mode: option expert_mode=1 !bash Hit (y) to confirm, flowadm add-flow -l NETWORK_INTERFACE -a transport=tcp flow1 flowadm show-flow -S Example: flowadm add-flow -l igb0 -a transport=tcp flow1 flowadm show-flow -S
This will give you a nice real-time status of your network throughput.
To remove the flow monitor, simply type in:
flowadm remove-flow -l network_interface EX: flowadm remove-flow -l igb0
Please be aware that there is some overhead involved in using flowadm for this purpose, as demonstrated by @kdavyd: