Joining DiskStation into Active Directory means that speeds over SMB may (and typically will) drop, but not a little, not a bit, they drop like 50% down. I found traces of this problem on StackOverflow, Reddit, Synology Community forums with some tips of which neither fixed that for me.
What’s Veeam doing here you may ask, well, that issue will hit you if you use Veeam to backup your virtual machines to SMB share using AD account to access that share.
Yes, that problem is ONLY occurring if AD account is used to auth against the share, if you decide to use built-in accounts of DS you should be OK.
Not ideal scenario for me as I hoped to use a single account, but having no other choice I added DSM account (make it own!) to Veeam and re-run backup job with speeds that are acceptable.