Welcome to TrueNAS Forums.
First, you did great trying to figure this out yourself, many people will not make this effort.
How were you moving file from Folder A to Folder B? Were you using a different computer to move these files? If yes, then we should try again, use a copy command and do an internal test to see if it is the NAS or the other computer you are using. I will need specifics to provide you a proper command. I need the full path of the folder from and to. Example: /mnt/mypool/folder_a and /mnt/mypool/folder_b.
I would recommend you upgrade to the current version of TrueNAS. That is the first step.
I’m not sure what you mean here. Zpool is a command in Linux.
Did you perform any stress tests? MemTest86+ and Prime95? Do you know what kind of NIC is there?
The problem you are having, I typically see with a RealTek NIC as it offloads all the work to the CPU. Maybe you have an Intel NIC but you should find out.
If you are having ZFS cksum errors, this is concerning.