FreeNAS does not come with iPerf3 (only iPerf2) as standard, which is a bit of a pain if you want to leave primary FreeNAS boot volume as pristine as possible.

The free OSS pre-compiled binary of iPerf2 for Windows can be found at https://sourceforge.net/projects/iperf2.

iperf -w 416k -i 1 -s
iperf -w 416k -i 1 -t 60 -c target.host

C:\Windows\system32>iperf -P 3 -w 416k -i 1 -t 900 -c 10.1.5.2
------------------------------------------------------------
Client connecting to 10.1.5.2, TCP port 5001
TCP window size:  416 KByte
------------------------------------------------------------
[232] local 10.1.5.1 port 54616 connected with 10.1.5.2 port 5001
[ ID] Interval       Transfer     Bandwidth
[232]  0.0- 1.0 sec   782 MBytes  6.56 Gbits/sec
[232]  1.0- 2.0 sec   747 MBytes  6.26 Gbits/sec
[232]  2.0- 3.0 sec   780 MBytes  6.54 Gbits/sec
[232]  3.0- 4.0 sec   750 MBytes  6.29 Gbits/sec