EtreCheck Report - iMac mid 2011

Performance:

System Load: 1.65 (1 min ago) 1.85 (5 min ago) 1.79 (15 min ago)

Nominal I/O usage: 0.94 MB/s

File system: 53.08 seconds

Write speed: 43 MB/s

Read speed: 62 MB/s

Your hard drive is running slow compared to the nominals for that drive model. Its should be doing read/writes over 100MB/sec and yours is doing 63/43 respectively (we have the same model iMac here with the same drive config). It is possible that, having removed CleanMyMac, you may see improved speed if you run EtreCheck again.

Avast must go as well.

I see traces of Kaspersky AV. Your government allegedly banished it from their offices over concerns with a Russian security product.

MS Teams currently requires a newer macOS version, so you may encounter some functions that no longer work. I saw two other posts about Team issues with older macOS versions in the last few days.

Crucial is a good choice for an internal SS drive. In your model, the read/writes will shoot up to ~500MB/sec making you think you have a new computer. However. I sincerely hope you bought the newer MX version and not the older BX model.

Note that the external USB SSD as boot volume option is not available to pre-2012 iMacs because they do not have USB3 ports.

Chrome is certainly having its way with your limited resources:

Top Processes Snapshot by CPU:

Process (count) CPU (Source - Location)

Google Chrome Helper (Renderer) (18) 25.85 % (Google LLC)

Google Chrome 10.10 % (Google LLC)

EtreCheckPro 4.28 % (Etresoft, Inc.)

launchd 2.56 % (Apple)

trustd (4) 2.38 % (Apple)

Just sayin'

What I see:

"More than one antivirus app - This computer has multiple antivirus apps installed."

You need to get rid of these; they're redundant, unnecessary and can and often do cause system disruptions.

All you need is the Apple AV that's built -in.

Budget constraints are not exclusive to the UK.

As you've already been told 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 documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

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 designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Uninstall CleanMyMac and Avast according to the developers' instructions.

You also have the utterly useless CleanMyMac installed. Users around here consider it to be malware. It provides nothing useful, but does a great job ruining the system.

Make a backup. Completely uninstall Avast and CleanMyMac. Reinstall the OS to fix any damage these apps may have already caused.

A secondary thought. Your Mac tops out at High Sierra. If that's what you're already running, then a Recovery Mode reinstall of the OS is fine.

I mention it because when you go the Command+R startup route, the Mac automatically wants to install the latest OS your Mac is capable of.

No. If you simply boot into Recovery Mode and choose to install the OS, it will only replace all OS items with new copies.

But, that's the purpose of making the backup, first. Just in case anything happens that shouldn't, you can still restore your system to its last state.

If you want a non-intrusive browser that also does a great job of blocking ads all by itself, try Brave.