Ok, so for testing purposes I have narrowed the backup down a single VM which is only 180GB in size as I cannot wait 20+ days for a full backup.
So last night I ran the backup on the 180GB VM which took a full backup of this in just under 8 hours. The processing rate was 12mb/s and the bottleneck was Source.
I ran it again today so it was only backing up the changes which obviously only took a short while (6 minutes) but the processing rate was 443 mb/s. I don't understand how it can run one backup at 12mb/s and then the next at 443 mb/s...surely it's running the same process - hitting the same target using the same agent?
As I said I am a bit of a Veeam novice so expect someone will tell me I'm wrong