How to clean up my computer

Macs do not need cleaning or any maintenance other than restarting them about 1x per week and keeping Mac OS up-to-date. However, you stated your Mac is up to date but is running slowly so lets see if there any obvious issues.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate towww.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

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1) Malwarebytes

Actively running Malwarebytes is part of the problem.

Temporally Stop Malwarebytes and test the iMac's preformance.

The macOS 13.7.2 update may have busted the current Malwarebytes version.

If you insist on actively running Malwarebyte Scans, check with the developers for an Update.

2) That 21.5" iMac model with 8Gb of RAM was made for light use.

In addition to the resource problem that you are having with Malwarebytes.

You have the full Microsoft suite, Google, Zoom and others are all Loaded and Running at the same time.

It would help a lot if you removed some or all of those App's from "Open at Login"

Then only open the App that you need to use and quit that App when your done.

3) That 21.5' iMac model is considered out-dated.

If it can not be upgrade to one of the 3 latest macOS versions..

Then it is time to consider a replacement ASAP.

den.thed's advice is good.

Also, you could consider upgrading the RAM through a AASP - although I'd argue the money is better spent on a new Mac.

You could also upgrade the hard drive - you currently have a fusion drive, which means a virtual disk made from a large mechanical HDD and a smaller SSD providing tiered storage. Having that replaced with a good sized SSD would improve things - but again, probably money better put toward a new Mac.

All in all, you have an almost 8-year old computer with specs that were probably OK in 2017, but not so much in 2024. A new computer is recommended. If that is not affordable, then finding an AASP who could upgrade your RAM and drive for a decent price (since the computer is vintage, it's no longer a matter of sourcing Apple parts, per se), is your next best bet.

Yup, I agree with the others. You have an 8 year old base model iMac that was never designed for more that basic web surfing, e-mail and very light productivity software (Word, Excel etc) My recommendation is to recycle the old computer with Apple and purchase a new Mac mini with 16GB of RAM and a 1 TB SSD. This will be a HUGE upgrade in performance over your current machine. You will only need an external display as you can use the old computer's keyboard and mouse. External displays start at <$100 and go up to about $1500 however for about $300 you can have a very nice display. So total cost about $1300 USD you have a new machine, 1 year warranty, legibility for AppleCare and you can also get 12 month no interest financing from Apple.