Even with the B3 stepping, the Intel i225 is not a fully reliable NIC. It can get very slow speeds for gigabit (eg limited to 75mbps-150mbps) and sometimes negotiate to 100mb instead of gigabit. It can also stop working entirely when dual booting Windows and Linux, requiring a power off.
- Disable checksum offload for TCP and UDP, IPv4 and v6. This can be found in Device Manager -> Intel i225-V -> right click -> properties -> advanced. This increased performance massively.
- Setting Maximum number of RSS queues to 1 (probably set to 4) is also meant to help