I faced this problem once when I upgraded to macOS Sierra back in 2017 and found the root cause by running some diagnostics software. This year (2021) I upgraded to macOS Big Sur 11.4 and it suddenly became super laggy again. Same symptoms, so I thought it might be the same issue. I think it is, because the same solution worked.
The solution is to go to System Preferences > Internet Accounts and remove any Google related accounts you have. Then, restart your computer and see if things work fine again. Then re-add your Google related accounts if you need them. I wrote slightly more details in my blog post here: https://www.nickang.com/2017-09-10-macbook-slow-after-upgrading-to-macos-sierra/
nickang
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