Plummeting performance

My FreeNAS experience was going swimmingly until last night and I'm struggling to figure out what's happening. Some quick background. After a fair amount of reading/research, I tested the waters with an older PC I had laying around (Celeron E3200, 4 WD Green HD's, 2GB non-ECC Crucial memory, etc.) and wanted to see FreeNAS first hand before making any more investments.

I configured a ZFS volume consisting of 2 mirrored 4TB drives and 2 mirrored 2TB drives. I'm not using any plug-ins and...

Plummeting performance