Well, for starters ZFS1 is wrong 
ZFS is the file system, and you mean RAIDZ1.
Suggested readings are Introduction to ZFS | TrueNAS Community and iX's ZFS Pool Layout White Paper.
I would suggest setting up a SSH terminal since you will have to do some commands… but SCALE apparently has fixed the WebUI’s Shell: up to your preference, though I still suggest SSH.
What’s the output of zpool status? When was the last time you scrubbed? How are you connecting the drives to the motherboard?
Also, I assume you mean 128GB of RAM instead of 12.