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
diskutil list
and post to your question with an edit (don't post in comments)