Thank you mijzelf for this explanation, much appreciated.
Anyhow in newer kernel there is specific config setting to enable smb2/3vsupport. My thinking was that it was required for enabling these protocols for the samba server. As I understand from your explanation this is not the case. So I learned something today. I will do a trial with an older kernel on a nwsp2 using samba 4x and see if I can enable smb3 protocol.
About 64k kernel influence on transfer speed
Mattmax did some speed trials with this kernel, see https://plugout.net/viewtopic.php?f=4&t=3635&start=220#p37419
Compared with the 4k kernel compiled from wd source code package a huge speed increase. For writing to the nas the 4k kernel only achieves 15 MB/s. That made me think that using 64k kernel improves transfer speeds. The pfe module that controls transfers from kernel to ethernet driver may also be the reason for the speed increase when compiling it with 64k setup. Just guessing here.