ix0@pci0:1:0:0: class=0x020000 card=0x7b118086 chip=0x154d8086 rev=0x01 hdr=0x00
vendor = 'Intel Corporation'
device = 'Ethernet 10G 2P X520 Adapter'
class = network
subclass = ethernet
ix1@pci0:1:0:1: class=0x020000 card=0x7b118086 chip=0x154d8086 rev=0x01 hdr=0x00
vendor = 'Intel Corporation'
device = 'Ethernet 10G 2P X520 Adapter'
class = network
subclass = ethernet

It's because of the new RSC support in the updated hn(4) driver which is apparently broken.
It only supports TCP to when you use OpenVPN (UDP) the traffic is unaffected.
See: https://forum.netgate.com/topic/169884/after-upgrade-inter-v-lan-communication-is-very-slow-on-hyper-v

Steve