for different synology models
this worked perfectly!
when copying/moving within (significantly faster)
and from nas to nas (more than double)
but it seems to depend on the DSM version!
do 13 was tested with the DSM versions
7.2-64570 to 7.2.1-69057 Update 4 (9 versions difference)
whether it behaves the same with older models with 6.x / 7.x
cannot be judged as many changes have been made to smb.
since the last 6.2 and the current 7.2, smb has been changed 5 times.
I have a small DS 419 Slim that I use for testing. It's running DSM 7.1.1-42962 Update 5. Using Dopus 13.1 with no settings changed. I moved 12.5GB worth of files (10 files) from 1 folder to another and it happened instantly with no transfer to local computer first. Hope that helps.
Copying a 11GB mp4 file to another folder within the same DS918+ (running DSM 6.2.4-25556 Update 7) using DOpus 13.1.1 resulted in a transfer rate averaging 100MB/s (200 MB/s peaks at times), over a WiFi connection averaging 25MB/s.
So yes, server-side copy offloading within the same (Synology) NAS appears to work in my case.