@Techniker_ctr
Regarding your installations is it UFS or ZFS, could you check top for an ARC entry (e.g., "ARC: 2048B Total, 2048B Header")?
I have three UFS pfSense instances (one bare metal, two VMs), and only one VM exhibits this issue. It displays an ARC entry in top and has zfs.ko loaded, despite being UFS.
Unloading zfs.ko temporarily removes the ARC entry, but it reappears shortly after. This behavior also occurred during OPNsense testing.
I've unloaded and renamed zfs.ko to observe the outcome.
HyperV CPU hvevent goes to 100%
Should a UFS machine have an ARC entry in top?
looking for the reason for sudden very high CPU utilization on network/zfs related process
"However this is not what is observed. What was observed is consistent with busy waiting. The storagenode process was constantly in the “uwait” process state. As if instead of pulling back, moderating bandwidth, sleeping more, storagenode instead loops at a very fast rate on system calls that fail or return a retry error, or just polls on some event or mutex, thereby consuming high CPU while effectively doing nothing (which is also observed: lower performance, lower overall IO, more missed uploads and downloads)."
Sound familiar?
/edit
Quick recap: zfs.ko loads due to a trigger in kernel like ZFS storage access. Is your storage dynamic or fixed? This instance experiences the most reads/writes, just a guess.