Can you go from a new OS to an older OS? I updated to the current OS and now my iMac is running slow and not working like it used to.

Backgrading is NEVER EVER a good idea. You should begin troubleshooting why the computer is running slow. Please read and do the following.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community

Downgrading macOS is possible and will often help speed up your computer. However, it will also open up your computer to more security vulnerabilities and make it less secure. Most older versions of macOS will continue getting security updates for several years. To downgrade macOS, create a bootable installer with the instructions bellow and then use it in recovery mode. All instructions you need to do that are on the article.

How to create a bootable installer for macOS - Apple Support

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for CleanMyMac & Bitdefender, and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac  and then restart normally. 

Before your post arrived, I saw from the Etrecheck report that I unintentionally had two AV programs installed. I uninstalled Total AV and my system was much better, like before the OS upgrade. After your post, I uninstalled Bitdefender as well which further enhanced my iMac's performance. Good to know that these programs are not only unnecessary but performance killers. Then cleaned up my MacBook Pro as well.

Many thanks!

I ran Etrecheck and have attached the text as suggested because after 12.3.1 upgrade, computer running very slow. Thanks!

I ran Etrecheck and have attached the text as suggested because after 12.3.1 upgrade, computer running very slow. Thanks!

Your welcome,

If you feel like the problem has been solved please mark the thread as such, doing so may help another user.