Memory Upgrade

Is your iMac slow? If so, adding Memory won't help.

However, to see why your Mac is slow, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

The 2012 21.5” iMac must be completely dissembled to upgrade the RAM.

It is expensive and you may have a hard time finding an Apple Authorized Service Provider to do the work.

Sadly given it’s age and the cost to upgrade, it might be best to consider using that money towards replacing the 2012 21.5” iMac with a new or at least newer model iMac.

Ask yourself these three questions:

• Is the computer slow to boot (start up)?
• Are many apps, even System Preferences, slow to load?
• Once slow-to-load apps are loaded and running, to they seem to work at normal speed?

If you give three "yes" answers, there is a 95 percent chance that you have a hard drive problem, not a RAM problem. Slow HD issue are rampant in 2012-2019 21.5-inch iMacs.

The is a non-invasive and modest-cost workaround available to the 2012 iMac that may give you some more time with it. Post back if interests in details.

+1 if you feel you need an upgrade it would be easier and wiser to simply replace the 2012 (remember its 10 years old now and VERY VERY slow) with a 24" iMac. Simply buy one with 16GB of RAM and the largest SSD you can afford.