I tried several tips to fix the problem, none had any effect.
I returned my iMac (2017) and Macbook Air (2015) to the Catalina macOS version, with completely recovery from zero. Everything is working fine (back) now.
What's going on with Big Sur, makes me imagine that Apple did it on purpose, to make Intel-based machines obsolete. Just like they do on iPhones.
PS.: I have another MacBook Air (with M1 chip) and Big Sur is so fast on that machine.
cirofeitosa
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Same issue here, on iMac and Macbook Air (both 2017).
Terrible slow after upgraded to Mac OS Big Sur 11.1.
iMac (Retina 4K, 21.5-inch, 2017)
Macbook Air (A1466 MMGF2LL/A)
When Apple will fix this?