Despite erasing the whole disc and reloading the operating system, after a while my iMac starts going slow and the spiral spinning wheel appears and slows down everything...

Have you run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

If you have download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

You have a couple of "bad boys" on your Mac. First and foremost is CleanMyMac. It's considered by most of the experienced users here to be akin to malware. Uninstall it according to the developer's instructions.

You can check to see if you've removed all of the files by downloading and running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For example for CleanMyMac software you'd do the following searches: 

1 - Name contains cleanmymac

2 - Name contains macpaw

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

The same goes for the Intego software. it's not needed and should be removed. Again use Find Any File to search for any file name that contains "intego".

Lastly, be aware that you have the slowest Mac that Apple offered at the time of purchase: 8GB of RAM and a 5400 rpm hard drive. Neither can be upgraded on the 21.5" model. You'll have to limit the open apps to one and close when finished with it. s

Greetings Leo D,

We're happy to help you with this slow performance issue you are having with your iMac. Great job in the troubleshooting done so far. Let's look a little more into this now. First, we want to check the hard drive space you have available. You can get the steps to do so here:

How to free up storage space on your Mac

We want to make sure that the start up drive has at least 30 GB or 15% of free space available.

Let us know what you find, and we can go from there.

Cheers!

Hello again LeoD,

Thanks for checking that for us. Now we need to see if there are any apps that are contributing to this slow performance issue. Find the steps to check that here: See how apps affect Mac performance, battery runtime, temperature, and fan activity

If you still have trouble after following the suggestions in this article, get in touch with Apple Online Support for further assistance: Get Support

Let us know if you have any questions.

Take care and be safe.