Welcome to the Apple Support Communities.
Please see the notes below from your EtreCheck report;
No Time Machine backup - Time Machine backup not found.
It appears that you do not have a Time Machine backup. If this is the case, you should create one. To start, you need an external hard drive. Here’s a few excellent ones sold by Apple:
You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support. Also, moving forward, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:
After you setup iCloud, turn off "Optimize Mac Storage" in System Preferences > Apple ID/iCloud. That way, all changes made to files in iCloud are also backed up to Time Machine. If you have this setting on due to a limited amount of storage on your Mac, that is fine. Just know changes won't be backed up to Time Machine, only iCloud.
Launch Daemons:
You should delete this Launch Daemon:
com.avast.av.uninstaller.xpc.plist
It can be found in:
/Library/LaunchDaemons
To go to that location, go to Finder > Go > Go to Folder, enter the location, and delete the Avast file.
Memory Clean.app
Delete Memory Clean: How to Delete Anti-Virus, Cleaners, VPNs,… - Apple Community. Mac computers do not need any sort of anti-virus, cleaners, VPNs, etc. The only thing they are good at is creating false marketing promises and taking consumers' money. They are all scams. In fact, anti-virus and other cleaners cause performance issues, security issues, and make macOS appear buggy. Your Mac is far worse with these types of Apps installed.
Mac computers have protection built-in, which is the best protection for your Mac. For specifics on macOS Security, see: macOS - Security - Apple and Protecting against malware in macOS - Apple Support. Mac computers also have built-in storage recommendations: Optimize storage space on your Mac - Apple Support. You can even use the built-in macOS Firewall: Block connections to your Mac with a firewall - Apple Support. Other than that, all you should do is restart your Mac approximately once a week and keep macOS up to date: Update macOS on Mac - Apple Support.
Google Chrome Helper (Renderer) (15) 1.28 GB (Google, Inc.)
I strongly recommend that you use Safari instead of Chrome. Safari is far more efficient, much faster, safer, and stable.
Performance: Below Average & APPLE HDD (Mechanical - 5400 RPM)
Other than that, the reason your iMac is slow is due to the 5400-rpm hard drive. While this hard drive found in your Mac is perfectly suited to light tasks, such as email, web browsing, etc., for more advanced tasks, and for users who want the maximum performance from their Mac, this hard drive does have its limitations. Luckily, you can use an external SSD as your startup disk to run macOS and all your data from that. It will make your Mac 10 to 30 times faster for storage, and in turn, make your entire Mac system much faster. For more info, instructions, and what external SSD to buy, please see: How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac - Apple Community. If you go the external SSD route, I would recommend the G-DRIVE mobile SSD in the 1TB capacity: G-Technology 1TB G-DRIVE mobile SSD R-Series Storage - Apple. These links can also be found in the User Tip linked above.
Another option is to replace this Mac with a new Apple Silicon Mac: Mac - Apple. If you go this route, it is generally recommended that you purchase 16GB of Memory with the largest SSD that you can afford. You can also use Apple Trade-In to get credit towards a new Mac: Apple Trade In - Apple.
Jack
