I have installed Monterey and it's slow and terrible, how do I remove this install?
I have installed OS Monterey and it's slow and horrible. How do I revert to my old operating system?
iMac 21.5″, macOS 12.1
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You would need to perform a clean install of macOS by first erasing the whole drive before installing macOS. Then during the first boot you can migrate your apps and user account(s) from a Time Machine backup you made before upgrading to Monterey. You can find links to various versions of macOS installer withing this Apple article which also includes instructions for creating a bootable macOS USB installer which you will need:
How to create a bootable installer for macOS - Apple Support
It may be better to try to determine the cause of the slowness. First try running the Apple Diagnostics to see if any hardware issues are detected.
Then to look for possible software issues run EtreCheck and post the report here using the "Additional Text' icon which looks like a piece of paper. If you give EtreCheck "Full Disk Access" the report will include more information such as a summary of the recent logs which may include useful clues.
You can run Disk Utility First Aid on the drive to see if any file system issues are detected. Within Disk Utility you may need to click "View" and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Click on the hidden Container or if you have a Fusion Drive select the "Fusion Drive" item and run First Aid. Even if First Aid says everything is "Ok" click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors listed, then you can try running First Aid from Recovery Mode, otherwise you will need to erase the whole drive and restore from a backup or clone.
Unfortunately the diagnostics don't detect all types of drive failures so it may be a good idea to check the health of the drive(s) using DriveDx. Post the report here using the "Additional Text" icon which looks like a piece of paper. If your Mac has a Fusion Drive, then please include a report for both the SSD and the hard drive.