iMac Retina 5k 27" Late 2015 RAM Expansion

Adding RAM most likely won’t help. For this reason, 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

Matt3o wrote:

Can I install 2 more 8GB memory modules, totaling 24GB

Yes

Can you suggest a specific brand or model of memory modules?

RAM upgrades from OWC and Crucial have never let me down.

https://eshop.macsales.com/shop/apple/memory/iMac

https://www.crucial.com/upgrades/apple

+1 on using Crucial.com or OWC (MacSales.com) modules.

Why do you want more RAM? Is it due to performance issues?

No problem, you're welcome.

Did you upgrade the RAM?

If no, what problems are you having?

If yes, what RAM did you use and what problems are you having?