following some test runs even with the support I have the strong doubt of being in this situation (if you need to specify the tests I have performed).
Q: Active full starts fast but then gradually becomes slower and slower. Target agent runs on Windows 2008 (not R2).
A: This is a known (yet undocumented) performance degradation issue on Windows 2008. Upgrade to or deploy 2008 R2 (or newer). In case of a CIFS repository you can force some other 2008 R2 server to be used as gateway/proxying. The issue itself can be demonstrated with diskspd if you set it to run for several hours, then observe how the write rate slowly goes down in Resource Monitor.vmware-vsphere-f24/full-backup-speed-decreases-as-it-runs-t10104-60.html
I tried hard but did not find documentation how to check this error / memory management bug.
on all Veeam forums the concept is "I've changed operating system and it works better"
since I seem to understand that for windows 2016 no one has yet reported similar problems as I can proceed to confirm or deny that problematic?