That's very odd....must be something in windows that fails to downgrade properly when the hardware changes.
FWIW, the rule of thumb for good performance is that no individual guest should have more than N-1 virtual cores, where N is the number of physical cores in the machine. For yours, that'd be 5. Using 6 can starve the host. I've found on my i9 that going over 4 can cause host stuttering too.
Thanks for the response and performance advice. I wish that was my biggest problem today 8^)
On my i9, Fusion sees 12 cores (6 phys and 6 HT) - If I select 6, are you saying that Fusion will use all 6 physical cores? I didn't know that!
I'm perplexed about this CPU count behavior too - it seems to be happening pretty early in the boot cycle - is there some HW dependency that's not being reflected properly in firmware handling?