iperf running on pfSense is NOT a suitable way of testing firewall throughput, as there is a significant difference between performance of traffic initiated or terminated on the firewall and traffic traversing the firewall. There are many suitable uses for iperf running on pfSense, but testing the throughput capabilities of the firewall is not one of them.
Uses for iperf on pfSense¶
Measuring throughput from the internal network to the inside of the firewall
Useful in scenarios where portions of the internal network are behind links slower than the firewall’s network interface. Examples include testing the throughput of a wireless network or private WAN network connected to a router inside the network.
Testing end to end throughput between two firewalls on the Internet
Anything else where performance measurement excluding throughput capabilities of the firewall are desirable.