I did find a solution to this once, it had to do with Windows Vista and 7 having a auto morphing MTU size, the Xbox 1 can only handle a MTU of 1500, but these Windows iterations will make it larger if it deems it can. If/When I find a solution I will post it here.
Ah Hah, found it on the XBMC site
[XBOX] HOW-TO fix Windows Vista and Windows 7 SMB Shares for XBMC - XBMC Community Forum
Hope this helps