Don't know if this will help but have you tried this:
Open your /etc/samba/smb.conf file and look for the line that begins with "socket options". The total line is probably something like this:
socket options = TCP_NODELAY
Now change the line to:
socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
And restart the Samba service.
According to the comments in the smb.conf file itself:
Most people will find that this option gives better performance. See smb.conf(5) and /usr/share/doc/samba-doc/htmldocs/Samba3-HOWTO/speed.html for details.
It's a long shot but who knows