What exactly is terribly slow (what benchmark do you use)?
And what kind of network do you have (1Gbit)? What kind of disks do you use for the proxmox server (model, vendor)?
GFS is a very clumsy way to organize backup, and only required if you have incremental/full backups.
Proxmox Backup Server give you automatic, fully duplicated backups, so you do not need GFS at all
(This is a great feature, not a deal breaker).
Why is "/var/tmp" a CIFS mount?
> INFO: filesystem type on dumpdir is 'cifs' -using /var/tmp/vzdumptmp281683_122 for temporary files
I would try to use a unix filesystem there (xfs, ext4, zfxs, nfs).