I dug out my MacBook Air (early 2015) yesterday. Literally. It had become wedged between the sofa an a unit next to it and I never realised as I don't use it much as it underperforms these days.
Still, I wondered what it would be like running MacOS Big Sur so I took the plunge last night. Colour me surprised. Where each new version of MacOS has gradually made the Air more and more sluggish but Big Sur seems to have reversed the trend. Apart from booting (which was never that great) everything feels snappier and apps load quicker than before.
I may start using it again if this is going to remain the way it behaves.