Poor overvall performance of Fusion 12 with old Windows as guests with Big Sur beta host…even with side channel mitigations disabled

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Had Fusion upgraded from 11.5.x (whatever the latest was) to 12.0 on my 10.5.6 Catalina MBP 16” earlier this week, did the same on my Mac mini late 2012 host as well, performance was identical to Fusion 11.5, and then upgraded my Catalina MBP to the latest Big Fur public beta, which went all.

I then noticed a significant drop in overall performance of some of my guests VMs. I turned off the side-channel mitigation feature in my VMs settings, as suggested, but no improvement noticeable.

Symptoms are :

– much slower graphics (windows being drawn on screen are noticeable)

– much slower “CPU feel” performance, including boot-up time, time to get to desktop once logged in, launching apps…

This happens with three of my guests, namely Win2K, Win98SE and WinXP. Strangely enough, my Win7 VM runs apparently as quickly as before, possibly even faster, and is now my guest OS that runs the quickest out of all 4, which certainly wasn’t the case before.

The exact same VMs (as they’ve been copied between my hosts) still run perfectly fine with Fusion 12 on my Catalina Mac mini host, thus using the older kext virtualization engine, so I’m guessing the issue is related to the API virtualization engine of Big Sur. For sure Big Sur is still in beta, but was wondering if this is to be expected or what solutions/troubleshooting you’d suggest?


Already tried :

– resinstalling Tools in said guests

– reinstalling Fusion 12 from scratch in Big Sur

– adjusting number of CPU cores/RAM in Win2k and WinXP guests



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