Your hardware is pretty old if I am seeing the correct info for the Intel Processor and the motherboard.
I am guessing the SSD is your boot pool / boot drive. How are all your drives connected? Are all six 4TB drives connected to your LSI SAS9211?
What is your pool setup? All six in one VDEV with Raid-Z1? Mirrored sets?
We will need the model number of all your hard drives. If they are all the same, just post once and note. We are looking to make sure your WD Red drives are CMR and not SMR for the way they record data.
Latest version of Core, is it TrueNAS-13.0-U6.2? or is it something slightly different? TrueNAS-13.0-U6.3 was just released Thursday
ZFS Primer in case you need to look up terms I used above.