The customer had observed very slow PVS target boot time.

Boot statistics on vDisk shows a long time to boot, over 600 seconds,  and boot time bytes read of over 2.5 GB.

Running DISM Clean up to cleanup the WinSxS folder:

• Dism.exe /online /Cleanup-Image /StartComponentCleanup /ResetBase (took several hours)
• Dism.exe /online /Cleanup-image /restorehealth (optional)

WinSxS folder after clean up reduced from 25GB to 8GB.

Boot time reduced to under 180 seconds.

Problem Cause

Using windows Performance toolkit, the boot process on the vdisk master was analyzed.
This showed CPU spikes of winlogon.exe reading data from folder “C:\Windows\WinSxS”
Contents of WinSxS folder was 25GB in size.