Ok guys... Morew test done with good news...
It seems that the problem come from the search zones used by the printer.
Here are the tests:
Target folder as \\NAS\folder\ takes 40 secondes to works.
Target folder as \\domain.com\DFS\folder\ that point to \\NAS\folder\ takes 40 secondes to works.
Target folder as \\NAS.domain.com\folder\ takes 2 secondes to works.
Target folder as \\WIN7\shared_folder\ takes 2 secondes to works.
It's more like a NetBios problem than authentification....