I am not familiar enough with kernel messages to say that is what is causing the speed throttling but kfence bug messages in IPFire in the past have been related to issues with nic drivers.
https://community.ipfire.org/t/bug-kfence-use-after-free-read-in-ipt-do-t/6888
I also note that you have i226 nics and those have been raised as having issues depending on the ethernet controller being used on the board.
For more experienced feedback probably need comments from @ms or @arne_f