Window Server 2012 throttling incoming IPv6 network file sharing operations

Question

Short version

• IPv4 download from server: fast
• IPv4 upload to server: fast
• IPv6 download from server: fast
• IPv6 upload to server: slow

Full version

I am copying files from a GBe desktop to a GBe server. The copy proceeds at a horrendously slow:

• 23 kB/s
• i.e. 0.2 Mbps
• 1% of GBe

But if I copy a file to a virtual machine hosted inside that same GBe server, copy proceeds as normal (512 Mbps = 50% of GBe):

I thought to myself, "What can Windows Server 2012 possibly being doing, that the virtual Windows Server 2003 is not?"

What about...

• It cannot be a cabling issue, as both transfers go across the same cables, through the same switch.
• It cannot be a drive speed issue, as the targets of both file copies end up on the same drive (a Storage Space parity drive).
• It cannot be drive write speed issue, as I can copy files locally around on the destination drive, at full speed (111 MB/s = 888 Mbps).
• It is not network speed, as I can download from the server at 994 Mbps.

As I was pinging both servers, I realized I was communicating:

• old server over IPv4
• new server over IPv6

Which then led me to my final, frustrating, test:

• Download from the server over IPv6
• and upload back to the same server over IPv4

And it's fast:

 Desktop                \\corsair (IPv6 identity)
+----------+           +--------------------------------+
|          | 994Mbps   |                                |
|          |<──────────|                                |  
|          |           |                                |
|          | 0.2Mbps   |                                |
|          |──────────>|                                |
|          |           |                                |
|        ┌─|<──────────|                                |
|        │ |           ┇                               ┇ 
|        │ |           |\\192.168.1.245 (IPv4 Identity) |
|        │ | 504Mbps   |                                |
|        └─|──────────>|                                |
|          |           |                                |
+----------+           +--------------------------------+

What is Windows Server 2012 doing that is causing it to fail so horrendeously on receiving IPv6 packets?

Note: I would have tried to test uploading over IPv6 to the virtual machine inside, but Windows XP/Server 2003 did not support File Sharing over IPv6 (cannot resolve the NetBIOS name).

Machines

• Desktop: Windows 7 64-bit
  IPv6 MTU: 1472
  IPv4 MTU: 1472

• Server: Windows Server 2012
  IPv6 MTU: 1472
  IPv4 MTU: 1472

Bonus: the MTU setting on client and server 1472:

>netsh interface ipv6 show subinterfaces

   MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
  1472                1  11739088006  581016212  Local Area Connection

The Questions

• Why is uploading IPv6 slow?
• Why is downloading IPv6 not slow?
• Why is IPv4 not slow?
• What setting must be adjusted to fix it?

Update

It's a litter over a year later, and the problem remains unsolved. Today i switched my desktop back to backup to the network server. It's a few hours later and the backup is still going. I remembered that Windows Server 2012 is slow when trying to send it IPv6 packets: