USB CLEAN INSTALL BIG SUR MBP 12.1. Minutes waiting for account switch, web pages etc. Tried updates etc. downloaded Catalina from Mac Store, USB, triple reformat, Catalina up and running, docs reinstalled, problem gone. Fast and productive..

Clean installed Catalina from USB, reformatted twice, back to Catalina, my lagg problems on 2015 MBP gone. FAST AND FURIOUS.