samba - Jul 2013 - Interesting slow Samba read and write problem - GnuCash

Interesting, but now reached the stage where I have to resolve it! Many
things have been checked and experimented with but nothing really has
provided the right answer.

The high points!
- Two Samba servers joined with an OpenVPN link over Internet. Worst
case bandwidth about 384Kbps.
- A WXP client on one of the servers LANs trying to access data on the
server at the other end of the link
- The data is a compressed GnuCash XML file around 180KB
- Reading or writing this data file (from within the app) takes around
3-5 minutes. (There are some extra things that GnuCash does of course)
- If instead of a GnuCash data file I try to read or write an Excel data
(220KB) file it takes maybe 10 seconds. Large file movements (hundreds
of MB even) work fine.
- If I mount the remote server share as NFS on the local server (with
sync/intr, tcp and udp etc) and access that as as local share, the
GnuCash read time is maybe 10 seconds, but the write time is about 30
seconds. (I didn't try to tune this. No doubt that could be a viable
workaround, but I only did it to prove GnuCash was "okay". GnuCash
speed
for r/w data on its the local server is fine)
- There does seem to be some kind of "resolution delay" before a file
movement starts. I am thinking that GnuCash actually reads and writes in
chunks that have some form of resolver time-out between each.

And;
- Debian GNU/Linux 6.0.5 (squeeze) Samba 3.56
- The Samba servers are separate domains, trusted with each other
- WINS is running on the remote end server
- Using tdbasm and idmap_tdb on both ends. The Linux UID/GID's are
exactly the same, but of course the winbind ones aren't.
- Resolved an OpenVPN fragmentation issue
- Both sites run bind and master/slave with each other
- Relevant bits of nsswitch.conf
passwd:         compat winbind
group:          compat winbind
shadow:         compat winbind
hosts:          files dns (use to also have wins)
networks:       files
- bind resolution/speed is fine. Tried pretty well all involved IP
addresses, and short/fqdn hostnames.
- There does seem to be a delay doing idmap/winbind resolution on the
affected files, but worst case that's maybe a second.
-  name resolve order = wins bcast

Like I said have tried lots of things. I am hoping someone will have
experienced this before and be able to point to the solution quickly. If
however anyone would like smb.conf or logs to review please ask!

Cheers Bob