I’ve found the cause !!!
This has been opened long time ago and it exactly points to my problem https://lists.samba.org/archive/samba/2015-April/190969.html
Hmm looks like earlier return for users with root permission,
so admin users would not go through SMB_VFS_GET_NT_ACL(),
which takes more time on permission checking.[1]
Non-admin users would go through SMB_VFS_GET_NT_ACL(),
and finally would reach getegid() and geteuid().[2]
The users that belongs to DomainAdmins groups are privileged regarding performance only because of this line in “resultant” smb.conf file -> admin users = “@domain admins” !!! So every share I made has this line and if I want to enable other users to use the share as fast as possible , other users of groups have to be added to this line i.e. -> admin users = “@domain admins” @other-group !!! Then those users belonging to this other-group have root-level access to the files in the share avoiding any ntfs permissions etc. I’m not happy with this solution but it is acceptable because the whole other-group can have rw rights anyway. … but there is another big BUT !!! … every time share configuration is being changed through gui , those custom entries are deleted. So , we have to use templates in order to preserve such customization … or maybe this value could be included in gui ? 
Obviously , access policy has to be moved up to share level … using ntfs permissions still has not been optimized so far .
Is it possible to define custom templates and assign them to different “types” of shares ? i.e.
So in this scenario , I will not be dealing with ntfs permissions in one “big” share , but I will create more different shares for different users/purpouses. In that case I will always have to extend the "admin users = " line in order to add users that will in realtime with this share
So the question wolud be how I could achieve this ? … through templates … where to tell this kind of shhare will use this template ?
Or is there some other way to , maybe, speed-up ntfs permissions usage ?
Thank you very much in advance
and
BR
Tonci