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I purchased this system in January, 2016. It says it is a "Late 2015" model. Apple Care told me it is the last generation compatible with non-matching RAM chip amounts so I left the Apple provided 8gb RAM (two 4gb sticks) in when I added 2 8GB sticks of Crucial RAM that is the EXACT recommended RAM specs! I even checked with Apple Care to be sure. "About this Mac" says "24 GB 1867 MHz DDR3" for "Memory". Anybody have an idea why it's momentarily "beach balling" MORE OFTEN now after adding the RAM?

Prior to adding the ram it was "beach balling" HARD if I ran memory greedy apps like Civ 6 Mac edition so I was forced to hold down the power button then but it ran fine the rest of the time.

To answer the drive type question in the comment, I bought the system with a 1tb non-Fusion HDD as that was the only drive I could afford. (Previous comment was incorrectly based on the 2007 iMac this system replaced.)

As requested the output of "diskutil list" is:

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_APFS Container disk1 504.0 GB disk0s2

3: Microsoft Basic Data Win 10 496.0 GB disk0s3

/dev/disk1 (synthesized):

#: TYPE NAME SIZE IDENTIFIER

0: APFS Container Scheme - +504.0 GB disk1 Physical Store

disk0s2

1: APFS Volume Macintosh 500GB - Data 133.9 GB disk1s1

2: APFS Volume Macintosh 500GB 11.3 GB disk1s2

3: APFS Volume Preboot 78.8 MB disk1s3

4: APFS Volume Recovery 528.8 MB disk1s4

5: APFS Volume VM 12.9 GB disk1s5

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    Do you have HD [Fusion] or SSD? Those last of the HD iMacs are just terribly slow. The only fix is a new SSD. – Tetsujin Jul 19 at 16:00
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    Well it is completely possible you have some RAM that is somehow defective or not compatible with the RAM already in it. Remove the Apple RAM and put the new RAM in its place. If it still happens try one stick at a time and see if the issue persists. If the new RAM is fine by itself in the Mac try the original RAM in the second slots. If that behavior comes back then the RAM you bought and the Apple RAM are (somehow) incompatible. Return the new RAM and buy another brand. – Steve Chambers Jul 19 at 16:09
  • Fusion drives came in the 2012 iMacs so I'm not sure how you came to the conclusion they didn't exist for the 2015 models. The answer below is actually good advice - start by running some drive diagnostics (the programs are free to analyze your disk, but cost money if you want to restore anything). Also, to get a clearer picture of your disk(s) installed, issue the command in Terminal diskutil list and post to your question with an edit (don't post in comments) – Allan Jul 19 at 17:47
  • You have a single 1TB HDD and in those particular iMacs, they were the spinning kind, not the SSD. Doug Masters answer is basically correct with the exception of not having the SSD (do you have the 21” model?). Either way, make sure you have a backup and make plans to get that drive replaced. I like Kingston, PNY, SanDisk and Samsung (all memory manufacturers). Swap with one of those and your iMac will be screaming fast when you’re done. – Allan Jul 21 at 21:00
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It sounds like you have a dying hard drive, not a memory problem - not that more memory is a bad thing! I did some research and according to EveryMac.com your iMac has both an SSD and a regular HDD. One or both could be dying. Most of my drives lasted around 5 years and that one is 5 years old now.

I found this post: https://apple.stackexchange.com/a/239723/377857 on how to fix it, but it seems like you have to remove the glass. I did a little more research (I want to do this to my iMac) and it seems like you have to cut the glue holding the screen. That scares me.

Since it is so difficult to do, I would probably use a program like Disk Warrior or Disk Drill (see them recommended by people all over this site) to test out the drive first. If it's not failing, why go through the trouble of opening it up?

If you do have to change it, it might be a good idea to get a pro to do this job. Maybe if I get more experience, I will try it myself, but not yet.

PS - In my experience, I've found that using matching RAM modules is the way to go.

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