Upon upgrading to Big Sur, I had the same issue on my MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports), 2.5 GHz Dual-Core Intel Core i7, 16 GB RAM. Lots of lagging on anything graphics related. Even simple text prompts or context changes were lagging on the order of 2-3 seconds.
Resetting the SMC fixed the issue for my machine!
A little odd, though, that my MacBook only began lagging after 2 weeks of using Big Sur. I hope Apple can patch this up soon. Quite the sizeable bug :/
jlovato
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