Tips from the field...
A single Proxmox host will only backup one VM or container at a time. Therefore if your hosts have an imbalance of VMs, one host will take much longer than the others.
^^^^ That is all true, but..... It really comes down to the sum total of the size of the VMs hard drives on each host. I had a three node cluster with many VMs and one file server. The VMs probably amounted to 1TB of actual, consumed storage, while the file server contained 10TB+ of files. The host that contained the file server would take well over 24 hours to finish, while the other two hosts would finish in an hour or two. This time difference was uncomfortable, but the fact that a backup job would breach the 24 hour mark was unacceptable. In the end, I had to split up the files across three files servers and pin one file server to each host. Only then would the backup jobs complete in a similar time.
^^^^ This is true, but..... Backup times of any single virtual hard disk can depend greatly upon how much of that virtual disk is actually used, and how compressible is the data. Therefore, YMMV.
