iperf3 between OPNsense and OpenWRT reached 900Mbit/s, so there was no bottleneck for all kinds of traffic. It only plummeted to 15Mbit/s when there were a lot of small packets to be transmitted. When I ran a speedtest on OPNsense itself, the CPU would hit 100% and I would see 50000+ Interrupts. Because OPNsense and consequently Proxmox were both connected to a 10Gbit/s Switch Port, OPNsense just flooded the OPNsense with as much traffic as possible and without buffering on the switch OPNsense was just drowning in packets. Instead of discarding all packets OpenWRT can’t handle, I think it’s a way more elegant solution to use Flow Control to throttle the transmission to what the router is capable of. I have a hard time believing that this would affect any other traffic (especially on my LAN), because the 10GbE NIC has 8 Queues which should handle different flows and TX/RX pause packets should only throttle the affected flows. Matter of fact, I just tested and I can still hit consecutive 770Mb/s transfer from my client to my NAS while running a speedtest on the OPNsense (TrueNAS is running on the same Proxmox host as OPNsense). And when disabling flow control all together, I only hit about 750Mb/s transfer speed… So I will stick with my current configuration, as it results in reliable, 0 packet loss transmissions with maximum speeds.