https://www.truenas.com/community/threads/what-is-the-freebsd-swap-partition.43563/page-2
What is the freebsd-swap partition. | Page 2 | TrueNAS Community
Someone in irc had a similar problem as you. It came down to the inefficient layout of 8disks and raid z2. Here is some info...
what is thefreebsd swappage 2partitiontruenas