My iMac 5K is a 2014, and I bought it in 2016 as an Apple refurbished unit. It just went out of AppleCare, but before it did I had the display replaced because it was showing some very serious image retention -- basically, anything that was on screen for more than a couple minutes, like the menu bar of an app, would be "burned in" when switching to something with a darker background. The first AppleCare person I got on the phone was terrible, was trying to tell me this was somehow normal -- and I basically hung up and talked to someone more reasonable who booked me an appointment to take it in. It did get fixed very quickly and without incident, though.
Now it's about 3 months out of AppleCare and the Fusion Drive, which was very fast and very reliable for a long time, seems to be slowing down considerably. The wear count on the SSD part seems quite high, and I'm thinking of getting rid of it and getting a newer iMac with a pure SSD. Currently grappling with the possibility of living with a 512GB internal drive and keeping my media (photos, music, videos) on an external SSD.
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If you don't want to pry it open, I think some people on here have had some success using a fast external SSD as a boot drive...
Did the new display solve the image retention issue? I get that on my 2012 iMac but it goes away quickly. In minutes. Never anything permanent. It was normal for this era iMac. I rarely notice it.