I have experienced this on two fresh TrueNAS install with syncthing being the only plugin installed.
Once ST is up and has a folder added the Host System is brought to its knees within a short time. GUI is unresponsive and service memory consumption grows extremely high (5gb on a 8gb system)
Scanning performance is working at relatively high/normal speeds but becomes unuseable quickly.
A user in the FREENAS forums suggest to disable “file watchers”. Are watchers not working in general on FREENAS / ZFS filesystem? or is this related to the ammount of files and folders being watche?
other people experiencing the same issue: see https://www.ixsystems.com/community/threads/syncthing-issues-crashes-freenas-and-slow-speed.83309/