Will downgrading Big Sur improve my computers performance?

Know that the model iMac that you have was the slowest iMac offered by Apple at the time of purchase: 8 GB RAM and a 5400 rpm hard drive. There's nothing you can do about the RAM but P. Phillips has given you an good solution to fixing the slow boot drive.

Also, 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.  

So uninstall Avast Cleanup Premium according to the developer's instructions.

I also noted that you haven't rebooting in 2 days. You should not let it go beyond about 2 days without rebooting. That clears out temp system and applications cache and swap files and will help in system/application performance.

eccletom wrote:

Will downgrading the system OS improve the speed without the need to boot off an external SSD?

That is a Good Question.

In reality, whether running Big Sur or Catalina, the same issue will remain - the 5400 RPM Mechanical Drive. That one thing is the bottle-neck no matter what older version is installed.

Use the External SSD will allow a few more year usage on this 6 year old machine.

You need to determine why your computer is running slow. Post an etrecheck report so that we can see what may be contributing to the performance issue.

With respect to using RAM (6GB of 8GB), of what value is unused RAM? Unused RAM is wasted RAM. Why pay for 8GB RAM if you don't want the computer to use 8GB RAM?

Post the etrecheck report.

The Biggest Culprit is disk0 - APPLE HDD ST1000LM024 1.00 TB (Mechanical - 5400 RPM) and will always spin too slow to effect the performance desired.

Possible solution is get a reliable external SSD Drive with USB and install the OS to the external drive. Then, in System Preferences >> Startup Disk, make the external drive the startup disk by default OR use the Option key to bring up the Boot Manager and choose external drive

In addition, at some point you installed avast. There are remnents of that third party app remaining on your computer. It is either still installed or was not properly uninstalled. You have to remove avast completely.

Welcome and thank you too.

As for the Drive in previous posting - look reasonable. The only aspect is the USB-C connection. Not sure the computer has the native usb-c female connections so an adapter would be required to convert USB-C to USB-A