my iMac runs very slow

We have the same model and, although slow to boot and launch apps, it is not slow once thing are running.

Your symptoms suggest a hard drive issue more than a memory issue. Drives wear out and yours is a dozen years old.

Are you running any useless software like anti-virus or so-called "cleaning" apps? Those are performance killers.

I have heavy memory usage.

What app or process is reporting this? Activity Monitor is the best way to check:

See memory usage for your Mac in Activity Monitor - Apple Support

How long has it been since you restarted the computer?

Download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. 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 Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

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