Grok gave this helpful explanation with info, but below this are other helpful resources:
https://grok.com/share/bGVnYWN5LWNvcHk_fcc5651e-d029-4c47-8ade-b60090030f31
FreeBSD Network Tuning Guide (OS OPNsense is based on):
https://calomel.org/freebsd_network_tuning.html
My example XML from within the NIC configurations I setup on the host within QEMU/KVM (the bold line is what to add, with 16 being an example number of cores to utilize for that network interface):
<interface type='bridge'>
<mac address='52:54:00:12:34:56'/>
<source bridge='br0'/>
<model type='virtio'/>
<driver name='vhost' queues='16'/> <!-- crucial for multiqueue + vhost -->
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>