Было у меня нечто подобное и осталась на диске сохраненная страница с тех времен, которая помогла починить тормоза. Саму статью в инете не нашел, поэтому сделаю цитату сюда. Упоминаются старые ОС, но чинил я это на 2012 и, кажется, помог метод 3.
Article ID: 888750 - Last Review: November 2, 2007 - Revision: 3.2
Slow performance when you try to access resources on your Virtual Server host computer from a guest virtual machine
SYMPTOMS
When you try to access resources on your Microsoft Virtual Server 2005 host computer from a guest virtual machine, the performance is very slow. When you try to copy a large file, the file copy may take minutes when you expect it to take seconds. Or, the large file transfer may not be completed. You may also experience that the guest virtual machine or other guest virtual machines that are attached to the same virtual network may disconnect from the virtual network.
This problem may also cause actual non-virtual network connections to the host machine to be disconnected. You may see an event log entry that is similar to the following event log entry :
Event ID: 2025
Source: SRV
Description: The server has detected an attempted Denial-of-Service attack from client \\computername, and has disconnected the connection.
CAUSE
This problem may occur if you have Transmission Control Protocol (TCP) segmentation offloading enabled on the host computer's network adapter. The Digital Equipment Corporation (DEC)/Intel 21140A chipset that Virtual Server 2005 emulates does not support TCP segmentation offloading. Therefore, the Virtual Server Network Driver Interface Specification (NDIS) filter driver must offload the TCP segmentation on the network adapter. This may cause slow performance and other network problems when you use Virtual Server 2005.
Method 1
Enable local area network (LAN) routing on the host computer, and then move the virtual network to the Microsoft Loopback Adapter.
Note To perform this procedure, your host computer must be running Microsoft Windows Server 2003.
To enable LAN routing on the host computer, follow these steps:
1. Click Start, point to Administrative Tools, and then click Routing and Remote Access.
2. Right-click your server name.
3. If remote access is already enabled, follow these steps:
a. Click Properties, and then click the General tab.
b. Make sure that the Router check box is selected, and then click OK.
If remote access is disabled, follow these steps:
c. Click Configure and Enable Routing and Remote Access.
The Routing and Remote Access Server Setup Wizard starts.
d. Click Next.
e. On the Configuration page, click Custom configuration, and then click Next.
f. On the Custom Configuration page, click to select the LAN routing check box.
g. Click Next, and then click Finish.
h. Click Yes if you are prompted to start the Routing and Remote Access service.
For information about how to move the virtual network to the Microsoft Loopback Adapter, see the "Using Microsoft Loopback Adapter" and the "Setting up virtual networks" topics in Virtual Server 2005 Help.
Method 2
Disable TCP task offloading in the registry of the host computer. To do this, follow these steps.
1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
3. Click Edit, point to New, and then click DWORD Value.
4. Type DisableTaskOffload as the entry name, and then press ENTER.
5. Right-click DisableTaskOffload, and then click Modify.
6. In the Value data box, type a value of 1, and then click OK.
7. Quit Registry Editor.
8. Restart your computer.
Method 3
If you do not want to disable TCP segmentation offloading on the whole system, and you want to only disable TCP segmentation offloading on the network adapters that Virtual Server 2005 guests use, you must not add the DisableTaskOffload registry entry that is described in Method 2. Instead, you can disable the task offload properties on the Advanced tab of the Properties dialog box of the network adapter.
Warning When you disable the task offload properties, guest virtual machines that are attached to the same virtual network may temporarily disconnect from the virtual network.
To disable the task offload properties, follow these steps:
1. Click Start, click Run, type ncpa.cpl, and then click OK.
2. Right-click your network adapter, and then click Properties.
3. Click the General tab, and then click Configure.
4. Click the Advanced tab.
5. In the Property box, click the Offload TCP Segmentation property.
6. In the Value list, click Off, and then click OK.
7. If you also have the following task offload properties in the Property box, you must repeat step 5 to step 6 to disable these properties:
o Offload Receive IP Checksum
o Offload Receive TCP Checksum
o Offload Transmit IP Checksum
o Offload Transmit TCP Checksum
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