I got the same problem with Proxmox 5.1 on a HP DL360p G8 (2x10Core/192 GB/HW-RAID 10 of 8x 960GB SSD) with LVM-thin.
I did 2 restores at a time from an external NFS storage to local SSD-RAID. All other VMs slowed down, webservers were not reachable, the gui also was not responding.
After a...