Windows 10 VM Slow After Big Sur Upgrade

I was on Parallels 16 and then upgraded to Big Sur from Mojave - afterward my windows 10 VM was painfully slow to the point of being usuable. I also noticed that in Windows the main drive was showing 100% active nearly all the time, plus the Mac drive showed a constant high read load - but not from Parallels....if anything, Finder was doing a lot of reads...

The VM was, however, an encypted VM. This had been no issue with previous versions of Parallels on Mojave.

But decrypting the VM remove all of the above symptoms. Not tried re-encrypting yet.

But why would this be?