NFS in bhyve VM mounted via bridge interface

I have a machine running FreeBSD 12.3-RELEASE with a zpool that consists 
of 12 mirrored pairs of 14 TB disks.  I'll call this the "storage 
server." On that machine, I can write to ZFS file systems at around 950 
MB/s and read from them at around 1450 MB/s. I'm happy with that.

I have another machine running Alma linux 8.6 that mounts file systems 
from the storage server via NFS over a 10 GbE network. On this machine, 
I can write to and read from an NFS file system at around 450 MB/s. I 
wish that this were better but it's OK.

I created a bhyve VM on the storage server that also runs Alma linux 
8.6. It has a vNIC that is bridged with the 10 GbE physical NIC and a 
tap interface:

[root@ss3] # ifconfig vm-storage
vm-storage: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 
mtu 1500
	ether 82:d3:46:17:4e:ee
	id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
	maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200
	root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
	member: tap1 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
	        ifmaxaddr 0 port 10 priority 128 path cost 2000000
	member: ixl0 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
	        ifmaxaddr 0 port 5 priority 128 path cost 2000
	groups: bridge vm-switch viid-ddece@
	nd6 options=1<PERFORMNUD>

I mount file systems from the storage server on this VM via NFS. I can 
write to those file systems at around 250 MB/s and read from them at 
around 280 MB/s. This surprised me a little: I thought that this might 
perform better than or at least as well as the physical 10 GbE network 
but find that it performs significantly worse.

All my read and write tests here are stupidly simple, using dd to read 
from /dev/zero and write to a file or to read from a file and write to 
/dev/null.

Is anyone else either surprised or unsurprised by these results?

I have not yet tried passing a physical interface on the storage server 
through to the VM with PCI passthrough, but the machine does have 
another 10 GbE interface I could use for this. This stuff is all about 
3,200 miles away from me so I need to get someone to plug a cable in for 
me. I'll be interested to see how that works out, though.

Any comments much appreciated. Thanks.