CleanMy Mac

No and No.

CMM is unnecessary to use on the Mac. It has been know to be overly aggressive and invasive in it's approach to "cleaning up" the Mac and causing problems with the macOS and user files. It is rarely recommended here in the Communities.

The macOS can take care of itself if you keep it updated.

If you find yourself in position where you question the performance of your Mac, there are two apps that I would recommend for troubleshooting. Malwarebytes anti-malware utility is one. The other is EtreCheck system diagnostic utility app.

A BIG +1 for D.I. Johnson's suggestions. We don't know which of the many sub-models of the 21.5-inch iMac you have, but we do know that easily 95 percent of reports here of "slow" iMac of that size made between 2012 and 2019 are not RAM issues or "cleaning" issues but rather the slow and under-spec hard drives Apple opted to using in entry level iMacs. No amount of RAM or software can improve that.

The is why I heartily second D.I.'s recommendation to post a EtreCheck report because if show us the important sub-model of your computer, the type of hard drive, and real-world performance data for your drive. And it does all that without reaveilgin any personal or security information.

You will need help posting the long report that results in the forums. For that, please see this User-submitted Tip:

How to use the Add Text Feature When Post… - Apple Community

As for all apps that claim to clean, mop, disinfect, scrub, or find you Mac a mate, they are useless, Why?

Because, cat-like, Macs clean themselves.

Apple developed elegant and automated self-maintenance routines over 20 years ago and embedded them in macOS. Without your having to worry or buy something, they take care of all the maintenance needs of your Mac in the wee hours of the morning. They even defrag the disk. Third-party apps interfere with this and can degrade performance. One cleaning app was found to automatically delete critical MacOS system files without even bother to ask!

Don't fall for fear marketing. Let you Mac be a Mac!

another NO!.

you may want to read Effective defenses against malware and other threats - Apple Community. it's a long article, but worth your time to read every word.

The same goes for run any "optimizing", "speed-up" or anti-virus apps on your Mac?