Same issue here. I'm trying to move appdata, domains and system to the array in order to convert the SSD to ZSF. All shares are set to high water but I experience the same slow througput. Also there is no constant transfer. Most of the time READS and WRITES show 0 and only occasionally low Mbit values.
Reboot did help a little but not much in my case (improved from 100KB/s to low one digit MB/s values).