iMac (Late 2015) lags; no visible cause in activity monitor

Welcome to Apple Support Communities.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to 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.

-Jack

Notes:

• Please re-enable automatic Time Machine backups. After all, that's the point of Time Machine.
• Java components are still installed. Follow the EtreCheck prompts to remove.
• Intego components are still installed.
• Delete AppCleaner.
• Delete any other anti-virus software.

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 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

-Jack

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Also, unless you're using a true VPN tunnel, such as between you and your employer's or bank's servers, they are useless from a privacy standpoint: Public VPN's are anything but private.  

That being said you've installed a bevy of "junk" apps:

2021-06-05 Mac Washing Machine Secure X9 (1.0)

2021-06-05 TotalAV (1.0)

2021-06-05 Avast Security (14.9)

2021-06-05 AvastHUB

2021-06-13 Avira Security

Uninstall all of those apps according to the developer's instructions.

When done boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and then reboot normally.

Hello, capslock286.

Maybe we can figure out what’s going on with your Mac.  It could be anything from a simple malware installation to a hardware failure. Some things are fairly easy to fix.

You should start with an EtreCheck evaluation of your Mac.

EtreCheck is a diagnostic app that gathers and reports important debugging information about your Macintosh. It is very highly regarded and meant to be used with Apple Support Communities to help others help you troubleshoot and resolve your Mac problems.

Please navigate to EtreCheck.com and download the free version of EtreCheck. Once you’ve run the app and created your report please post it with your reply to this message. 

You must upload the full report. For instructions on how to attach your EtreCheck report to your reply, please click  How to use Add Text when posting…

We can use the EtreCheck report to look for the more obvious things that may be causing your problem and advise how to correct them.