After I've installed Hyper-V role system boot time rose from about 1 minute to about 10 minutes. I have i7-3930k CPU and Intel DX79TO motherboard with latest BIOS installed. I'm on Windows Server 2012 Standard.
The problem is not about guests, its about host OS. After computer BIOS finishes its actions and I select Windows Server in boot menu, screen goes blank and HDD indicator is lightened for several minutes. After that Windows logo appears and system finishes to boot in about 20 secs. This several minute blank screen delay disappears when I remove Hyper-V role and reappears when I re-add it.
The same behavior is observed on Windows 8 which is installed on another hard drive. When I install Hyper-V in it, the same several minute blank screen idling appears
I've tried Microsoft community forums with no success on my particular issue. All what they said to me so far is to use Windows 7/2008R2 instead of Windows 8, because these OSes don't have that problem on my machine.
But I'm developing software for Windows 8, which is much more comfortable from developer tools point of view to do from Windows 8 itself. I need Hyper-V to run software in emulator which comes with Windows 8 SDK.
I also develop some of system software, particularly virtual disk driver, which, unfortunately, can't be debugged in virtual machine. It currently relatively often makes host OS unresponsive, which forces me to reboot, which faces me the 5+ minutes boot time problem. (Just in case: the driver is not related to the boot time problem, as it repros on clean Windows 8 + Hyper-V installation).
Currently, I worked around this problem by creating a separate boot entry for my Windows 8 with disabled Hyper-V, so when I work on driver, I use that entry, and when I work on Windows 8 applications, I use boot entry with Hyper-V enabled.