My iMac is too slow

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

Notes:

• Delete McAfee and all other McAfee components.
• Delete any other Anti-virus, anti-malware, or cleaning apps.

Other than that, the reason your iMac is very slow is due to the 5400 rpm hard drive. Very slow. If you use an external SSD and run macOS from that, it will make your Mac 10 to 80 times faster for storage, depending on what external SSD you buy. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community. It appears a 500GB external SSD should be enough room, but a 1TB might be better for "future-proofing" for future storage needs.

Cheers,

Jack

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.

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.

Copy the report

and use the Additional Text button to include the report in your reply.

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

It is not possible to answer that question without knowing certain basic facts. Did it just become slow? If so, what might you have installed or changed? What kind of hard drive do you have? Do you have any antivirus apps or Mac cleaner apps installed? Do you ever restart your computer? We here would recommend that you download, install and run the free app Etrecheck. When you run it, be sure to enable full disk access. Save the report it provides and then respond and post it below under "Additional text". This report provides a comprehensive view of what is going on in your Mac. Then someone here can review it and offer some input.