A few weeks ago after an in-place upgrade from Windows 2012 to 2012 R2 Hyper-V started behaving poorly, especially during the VMs power on. All of them were spending abnormal duration on “Starting (10%)” phase – at least several seconds. Quite annoying problem which definitely required resolution as on the same box with win2k8 R2 Hyper-V worked like a charm – a VM used to power on in less than a second.
After troubleshooting through drivers compatibility and processor performance counters the root cause was found in the enabled Intel C-State Tech feature in BIOS. Disabling this power saving feature returned back the expected performance of Hyper-V.