Just got off the phone with tech support. I was experiencing the same lag issue after upgrading my machine from Catalina to BigSur.
We first shutdown the machine and then restarted the machine in Safe Mode. (Hold Shift - Powered up - Let go of Shift once the Login screen appeared.)
Interestingly enough, we actually experienced some even slower activity when we explored machine functions in Safe Mode. Apple applications were working as they should - where they were not previously.
We shutdown down again. This time we reset the SMC. (Hold down Control + Option on left, Hold Shift on right. Hold for 7 seconds. Add holding the power button for another 7 seconds.)
Machine now seems to be functioning much better.
manngroup
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