I ran into this issue as well. An interesting tidbit that might help others.
When the /mnt/user disappeared, I always ended up rebooting because neither docker or kvm would work properly after bringing the array off/online. Upon reboot, the parity check would be triggered but the rate was very slow. Seeing > 15 MB/sec was rare. All the benchmarks on the drives showed over 150 MB/sec or higher for each drive so it was rather confusing.
Disabling NFS sharing on each of the shares and manually putting the nfs shares into /etc/exports using /mnt/user0/... resulted in an increase of parity check performance to 200 MB/sec. Something is definitely amiss with what the GUI is doing to the shares with NFS.
YMMV as always