Thanks for trying!
I have been investigating this further and found out this:
Upon disabling Oplocks for the folder on the NAS where the ISOs and mkvs are stored in, the smbd CPU load issue seems to be gone, i.e. load is around 0-5%, so it looks like Oplocks caused this.
Sadly, I know very little about SMB/Oplocks/Samba in general, so I’m not really sure how Oplocks could be related to this, since it’s supposed to improve performance.
