Hmm i think that was only a temporary effect, my experiences after further testing are at copying large files to Client (Synology is at latest Software 6.2, Windows 10 at latest 1803):
- Synology with SMB2 or SMB3 and Eset: stuttering
- Synology with SMB2 or SMB3 and Defender: no stuttering
- Server 2016 and Eset: no stuttering
-> Conclusion: something goes wrong in combination with Eset and Synology SMB sharing ...