Virtual machines (VMs) that are running Windows Server 2012 R2 may experience poor performance if the VM storage stack is in emulated mode. Additionally, you may experience a severe decrease in local resource VHD (temp disk) I/O performance.
This issue occurs because of a recently discovered bug in Windows that's exposed when the image that's being provisioned has phantom IDE devices. When this issue occurs, the storflt driver is uninstalled from the VM storage device stack unexpectedly. This causes the VM storage stack to use the slower path that uses emulated mode each time the VM restarts.
The driver remains uninstalled and isn't re-installed during successive restarts.
This issue is resolved by update 4057903 that is available through Windows Update. For more information, see the following Microsoft Knowledge Base article:
4057903 Hyper-V integration components update for Windows virtual machines