Super Slow Boot

I have a Windows 10 WIMBoot system which is taking ages to boot.

It literally takes between 5-10 minutes to boot.

The time lost during boot cannot be measured. It doesn't show on any boot performance tracker.

Once the actual boot process starts (dots spinning). booting is near instantaneous.

However between the OEM logo being shown and the dots spinning, it can take anywhere from 5 to 10 minutes.

This time is literally lost in a black-hole!

Anyone have any thoughts on what the matter might be?