Slow LAN connections MacPro(new) + Mavericks

Question

Hmm.. We run IPv6 here among all machines in addition to IPv4.
Mavericks preferentially runs most services over v6 if you have it
and Bonjour goes for a v6 address too when connecting.
New MacPro to old MacPro (2011), MacBookPro etc. Mavericks on all.
Problem
1. SMB2 file sharing was transferring at around 300Kb/s
2. JollysFastVNC was almost unsuable, screen lag was awful.
3. Transers failing due to timeouts etc.
Tests
Forced SMB2 to SMB1, no change. Tried SFTP, SSH, same.
tried switching to link local address (fe80::) with no difference.
Everything is assigned a valid routeable v6 address via router
advertisement, etc.
Fix
Forced Mavericks services such as SMB to use IPv4, now see ~100MB/s between machines on
a GigE switch with a tiny cpu load, like 4%.
Note
What's odd is that IPv6 connections to outside (ie to the net) are blazing, we
see > 57Mb/s over Comcast Business class to Google, Facebook, for example.
This all worked peachy pre Mavericks (LAN wise).
Conclusion
1. IPv6 Mavericks LAN throughput is somewhat broken.
2. Force file sharing, VNC etc to use IPv4 by using the remote sides v4 address when
connecting.

Answer 1

Try a factory reset of the Home hub by pressing a pin into the recess on the rear.
My advice would be to uninstall BT Desktop Help and see how you get on. You don't need Desktop Help it and while it might not be the cause of your problems it is known to cause problems on some computers. To uninstall it follow these instrutions:
Uninstalling BT Desktop Help
Go to Start
Select All programs
Select BT Desktop Help from the list, then Uninstall BT Desktop Help
Follow the onscreen wizard through to completion
Check the Task Manager Processes tab (Ctrl + Alt + Del -> Task Manager) and "End Task" if any of the following are listed:
BTHelpNotifer.exe
McciTrayApp.exe
McciServiceHost.exe
If you see McciCMService.exe, 'stop' this in Services (Control Panel -> Administrative Tools -> Services)
Open the Registry (Start -> Run -> Type "Regedit") and delete the following directories:
HKEY_LOCAL_MACHINE\SOFTWARE\Motive
HKEY_CURRENT_USER\Software\Motive
(If you have a 64-bit computer) HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Motive
Clear the Internet Explorer Cache and Cookies (Within IE -> Internet Options -> Delete…)
Delete any leftover Program Folders from Windows Explorer (Windows Key + E)