Comment 67 for bug 1785171

FYI a kernel bisection may be outside the skill set of your average user and chances are much lower without detailed instructions. Please don't shoot the messenger.

I too have a Asrock motherboard but a ASRock Z370M-ITX/ac. I had a lot off trouble getting my I219-V NICs to behave on Ubuntu.

I started on 18.04 and eventually went to 19.10 with the Intel e1000e 3.6.0-NAPI driver and a bunch of commands to get good performance.

File transfers were severely impacted but it is immediately obvious in iperf3 when the card is not performing properly.
Tweaking several of these options added several hundred Mbps to transfers giving a consistent 60MB/s over SMB but I'm still getting half the throughput of my Windows 10 workstation when talking to the same Ubuntu server. Given my Ubuntu client storage is 5X faster the cause is unknown.

I'm curious if anyone has figured out if the firmware version on the I219-V NIC is updateable and how. It's odd the same chip users different drivers and shows different firmware revisions.

$ ethtool -i enp2s0
driver: igb
version: 5.6.0-k
firmware-version: 0. 4-1
$ ethtool -i eno1
driver: e1000e
version: 3.6.0-NAPI
firmware-version: 0.2-4

It's really odd to be using two different drivers for the same NIC:
sudo lspci -v | grep -i ether
00:1f.6 Ethernet controller: Intel Corporation Ethernet Connection (2) I219-V
 DeviceName: Onboard - Ethernet
 Subsystem: ASRock Incorporation Ethernet Connection (2) I219-V
02:00.0 Ethernet controller: Intel Corporation I211 Gigabit Network Connection (rev 03)

I have this in my rc.local now:
ethtool -A eno1 autoneg off tx off rx off 2> /dev/null || :
ethtool -K eno1 gso off tso off 2> /dev/null || :
ethtool -A enp2s0 autoneg off tx off rx off 2> /dev/null || :
ethtool --set-eee eno1 eee off 2> /dev/null || :
ethtool --set-eee enp2s0 eee off 2> /dev/null || :

1st line turns off flow control, 2nd turns off offloading specifically gso and tso, the last ones turn off a energy efficient Ethernet option.
Post tweaks I consistently get 935-958Mbit/s in perf3.