iMac (Retina 4K, 21.5-inch, 2017) 16 Go memory, can I upgrade to 32 or more ?

Correction: Memory (RAM) in 2015 iMac 21.5-inch models is non-removable. Memory in a 2017 21.5" iMac is technically "removable." It simply is not user-accessible.

Doing anything with RAM in 21.5 inch iMacs made between 2012 and 2019 requires virtually a complete gutting of the computer to the empty-case stage. The task is so difficlut and time-consuming that Apple suggests that some Apple Authorized Service Providers and independent service providers may not work on sealed-case iMacs like yours.

If your computer is slow, there is a 99 percent chance it is NOT due to having 16GB RAM. That is more than enough for most home and office users. I have that amount in a 2017 iMac 27 and it has never tested as being starved for RAM. Take this little quiz for owners of slow 21.5-inch iMacs:

• Is the computer slow to boot?
• Are apps slow to load?
• Once loaded, do apps seem to run OK?

Three "YES" answers say your issue is the the slow and under-spec base hard drive Apple fitted to entry-level iMacs from 2012 through 2019. It is mechanical, a slow SATA 3GB model attached to a fast SATA 6GB drive bus (like a Yugo engine in a NASCAR racer), is rated at only 5400RPM, and is a laptop-class drive, not a desktop-class.

If that is the case. NO AMOUNT of extra RAM can speed your computer. You need to address the drive.

We can confirm the diagnosis if you post a system configuration report. We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck Pro, available here:

https://etrecheck.com/en/index.html

The free version will do nicely although the app is fully worthy of our financial support.

We can see hard data about drive performance and RAM usage, plus see of there are any software conflicts. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find) 

Then click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown:

Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

⚠️ NOTE: Dec 2018 changes to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Additional Text” window.

The report is comprehensive while preserving your privacy. We can usually confirm a diagnosis and make effective data-supported recommendations with only one more post from you. Otherwise we end up in a protracted game of :20 Question: with you.

Looking forward to your report.

No, unfortunately, it’s not removable.