PVE version, as restore is PVE side.
PVE 8.4 require updates to allow "Restoring from Proxmox Backup Server now fetches multiple chunks concurrently in each worker thread.
This can improve the restore performance in setups with fast network connections and disks.
The number of concurrently...
After you discover how it works under the hood, you will follow the recommended way : Local PBS then WAN PBS Remote Sync from.
Give a try , I bet on no more QEMU guest agent stopped with a local PBS.