everyday we backup ~3300 VMs at the same time, including a synthetic full every week. In 12.1 this was not working very efficiently and we asked Veeam to optimize this. Back then (case 02476388) Veeam was sure our Network was the issue and not much could be done.
But now with 13.1 (without any network changes) basically all issues are gone. Synthetic jobs do not really run longer than normal incremental runs.
Some numbers:
Average Backup Window:
12.1: 320 minutes
13.1: 93 minutes
Average Time per VM:
12.1: 10 minutes
13.1: 2,1 minutes
Cumulative VM Task time:
12.1: 579 h
13.1: 124 h
All that while todays backup was 27 % larger than the average backup.