I just spun up a VNET jail (so it should be essentially using the same network stack and networking isolation level as a bhyve guest would) and tested with iper

I just spun up a VNET jail (so it should be essentially using the same network stack and networking isolation level as a bhyve guest would) and tested with iperf3 and without any tweaking or optimization and without even using jumbo frames I'm able to get 24+ Gbps with iperf3 (32k window size, tcp, single stream) between host/guest over the bridged and virtualized network interface. My test hardware is older than yours, it's a Xeon E5-1650 v3 and this is even with nested virtualization since the "host" is actually an ESXi guest running pf! But I think you might be right about something because, playing with it some more, I'm seeing an asymmetry in network I/O speeds; when I use `iperf3 -R` from the VNET jail to make the host connect to the guest and send data instead of the other way around, I get very inconsistent results with bursts of 2 Gbps traffic and then entire seconds without any data transferred (regardless of buffer size). I'd need to do a packet capture to figure out what is happening but it doesn't look like the default configuration performs very well at all!