So, I may have tracked down the issue. I turned off my syncthing jail. No slowdowns or errors in the last 24 hours. I will give it another day or two and see what happens. Worst caser I will nuke the jail and start over.
One of my attempts to fix things was a fresh FreeNAS/TrueNAS on a new flash drive. That did not fix anything. But, that did not change my jails since those are installed on the storage volumes.
So far things are still running smoothly with Syncthing off. I will probably try just reinstalling the plugin. If that fails, totally wipe it from existence. Including the share it is tied to. Nuke it from orbit. It's the only way to be sure.