Cifs takes too long during authentication

Dear All,

I use the following command to mount a remote MS Windows directory
(through a VPN connection):

mount -t cifs -o iocharset=iso8859-1,user="?????" //??????? /mnt/iso

It works fine but it takes more than 3 minutes to complete the
authentication process. Any ideas about how to speed up the
authentication process?

Thanks in advance,

Paul