How I can speed up my iMac?

In addition to Jack's great info...

We need to see what all is running, a report from this will not display any personal info...

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

Hi there!

In order to start diagnosing your Mac, we first need to know some information about it. 

To start, using this Apple Support article as a guide, Get system information about your Mac, let us know what year your Mac is, the model, and what version of macOS you are currently using.

Next, please upload a screenshot of your Mac’s storage information. In the Finder, choose Go > Utilities, then double-click the Disk Utility icon. (Disk Utility is located in the /Applications/Utilities folder.) Alternatively, click Launchpad in the Dock, type Disk Utility in the search field, then click the Disk Utility icon. Once Disk Utility is open, take a screenshot of the information it automatically opens to, which will be your startup disk. Next, select View > Show All Devices in Disk Utility and select the top-level, non-indented, disk entry. In most cases, this name will be one of the following:

• APPLE HDD…
• APPLE SSD…
• Fusion Drive

Then, take a screenshot of the newly selected entry. Upload all screenshots here. 

Next, it is recommended you Use Apple Diagnostics to test your Mac in order to identify any potential issues. Let us know the results of this test. 

Finally, list any anti-virus, anti-malware, cleaning, or VPN software you have installed. 

Jack