Possible same issue here: slow SMB transfers from Mountain Lion over VPN to Windows Server with no answer.
On the local LAN SMB file browsing is OK. It's slower than a Windows client, but manageable.
However, over a VPN (Cisco SSL/IPSEC Anyconnect), it bogs down from seconds to minutes, despite the VPN from the user's house being faster than the MPLS links at the office.
The web has various fixes but I didn't want to simply start guessing at which to choose first...was hoping for some guidance from the SU community.