getting 'beach ball' quite often after installing macOS Big Sur v11.6.1

If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage compared to the internal HD. For more info, instructions, and what external SSD to buy, please read: https://discussions.apple.com/docs/DOC-250003583.

+1 on posting an Etrecheck report. We can neither see nor touch your computer in this venue, yet the report shows us both hardware and software issues that can slow a Mac WITHOUT revealing any personal information. It its the single-most effective tool for us to help with Mac problems in a text-based environment.

See this user-contributed tip to see how to use the "Additional Text" forum function to post the entire report:

How to use the Add Text Feature When Post… - Apple Community

Even without the report, history here teaches that there is about a 95 percent chance that a slow entry-level iMac 21.5-inch made between 2012 and 2019 does not have a RAM issue but rather the underperforming mechanical hard drives Apple used. The 2012-2019 models are demonstrably slower than entry-lelver 2.15 iMacs fomr 2011!

+1, so +2, on EtreCheck

More information is needed to know precisely what is the cause. It is true that Big Sur uses more RAM than earlier operating systems. The first question is how much RAM do you have installed? You can check in the About This Mac window, accessed under the Apple logo. Do you have any anti virus or Mac cleaner apps installed? They can seriously slow down a computer and are unnecessary and should be removed. We would recommend that you download, install and run the free app EtreCheck. Be sure to enable full disk access. It will generate a report you can post below under Additional text. That will provide a comprehensive overview of your computer's operations and likely will explain what the problem is.