Big sur 11.2.2 very lagging on 2019 Macbook Pro
Here is how I would back date a newer Big Sur system to 11.2.1
If you have a Time Machine Backup from before you ran the upgrade. That means you can erase the Mac - get the installer for 11.2.1 and then erase / install in a fairly straight-forward manner.
Once 11.2.1 is running, migrate data from the Time Machine snapshot or backup interval before the upgrade (or whatever your custom backup procedure entails). If you don't have a backup - make one now so you can hand migrate some of the easy apps and files that are compatible with a downgrade - some apps like Photos databases don't necessarily work backwards so you'd need to handle those errors and issues one by one.
Also - Catalina may have made a snapshot of your system that’s usable as a backup, so you would just boot to recovery and restore the system to that shapshot and not even need to worry about an installer.

https://support.apple.com/en-us/HT204015


And in macOS High Sierra or later, another snapshot is saved before installing any macOS update.

If you need to make a bootable 11.2.1 installer, here's how:

Delete any Install macOS Big Sur apps you have in /Applications (or copy them to an external drive)
Open terminal.app or another tool to get to the command line and issue this command


softwareupdate --fetch-full-installer --full-installer-version 11.2.1

Once the installer is there, follow this guide to make an installable USB for 11.2.1

https://support.apple.com/en-us/HT201372

Then follow this guide to erase install.

https://support.apple.com/en-us/HT208496