I'm seeing the opposite, Carl, but I upgraded my 2012 Mac mini to a 2018 Mac mini in late 2019. The 'new' hardware has a 3.2 Ghz 6-core i7 and 32G RAM vs the old system's 2.7Ghz 4-core i7 and 16G RAM. I did this because I was concerned that it was reaching the limits of what I would be comfortable with using the older processor and less memory, and today's 50 Mpixel raw files from the Light L16 and Hasselblad 907x. LR Classic v10.1 is likely not optimized for Big Sur on older hardware, I suspect it depends on what exactly you're working with as to how much of a performance hit there might be.
You should see if LRC's "use graphics processor" option (Lightroom Preferences, performance page) is compatible. Some folks have said that it is not with their 2012-2014 Mac systems, but if it is, the performance is often nicely enhanced with that option enabled.
G