installd found in Activity Monitor using almost all memory RAM. Don't know what is installing :-(

Welcome to the Apple Support Communities.

What you are looking at in Activity Monitor is disk (hard drive) usage, NOT the RAM. "installd" is a storage process that accounts for anything that has been installed on your Mac. This could be an update, an App installation, etc.

This would likely not account for why you Mac is slow. For this issue, 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:

• No Time Machine backup - Time Machine backup not found. • It is very important to have a backup.
• Delete NordVPN, unless it is really needed. VPNs make you connection less secure.

Other than that, the reason your iMac is very slow is due to the 7200 rpm hard drive. It also might be starting to fail. 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.

Cheers,

Jack