I'll try to fill in as I can, but here's the basics
We have a VPN (I believe running openvpn) between a site in singapore to a main site in london, with the main site.Its a purely ipv4network.
We have a windows 7 box (in sg) which we use to retrieve a file from a centos 6.5 server running smbd Version 3.6.9-167.el6_5 (in london). Browsing the folder seems workable (few seconds to open a folder) but It takes something like half an hour to start moving a 16mb file, and takes a few hours to download. The issue is replicable with a fresh windows install on our side.
Using a centos box on the same network segment as our system seems to work fine.
Historically there was a Windows xp system doing the same role and it supposedly worked fine. The graphical gnome "connect to server" option works fine as well
We had a period of pretty bad (>300ms latency) between hosts but but currently its sub 200ms and we have the same issue
We've tried forcing the windows 7 box into smb 1 mode to see if it helps.
I don't have the wireshark logs (and am not really at liberty to share them), but I distinctly recall a significant amount of out of order packets when I checked.
How do we work out why its taking so long to initialize a transfer and how can I speed this up? We can't really do much on the UK end, but we can test possible solutions on the singapore/windows side.