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Long story short:

If I configure discs on the machine as a RAW partition so Hyper V works directly with them to increase performance then the machine takes minutes to boot with the blue circle going in rounds. If I put the discs offline then it boots in seconds.

Any ideas on how to avoid this?

Ignacio Soler Garcia
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The physical disk in pass-through mode should be accessed only by single OS (either host or guest) at the same time to avoid data corruption. That’s why physical disk should be marked as “offline” in host OS (Microsoft requirement). Actually, starting from Windows Server 2012R2 there are no noticeable benefits using RAW disk versus “fixed“(not dynamic!) vhdx. The performance is the same or near the same, but vhdx gives more flexibility with migration, extension and backups. Here a good article with explanation - https://www.altaro.com/hyper-v/hyper-v-pass-through-disks/

P.S. to improve overall Hyper-V performance the following might be helpful - http://www.hyper-v.io/several-tips-hints-full-throttle-hyper-v-performance/

batistuta09
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    That makes sense, the host was not accessing it as it does not support RAW partitions but someone tries to do it and probably that generates the slow boot process. – Ignacio Soler Garcia Jan 28 '22 at 07:58