I appreciate your tests. But to check true routing/NAT performance, testing should be done with two computers, one connected to the WAN interface (running as server) and one to the LAN interface (client). In different subnets and with Masquerading in the WAN zone of the firewall. This test will give real/accurate results to those that will be obtained using it as a router in production.
Launching iperf3 on the router itself (as client or server) is a huge bottleneck, due to the extra processing that the CPU must do for iperf3. Also, routing/NAT capability is not taken into account. The computer running the opposite role (server or client), is on the same network that is running iperf on the router (so no NAT is involved).