Worst. iMac. Ever. Actually...

[Fishrrman]

OP:

Buy an external USB3 SSD, plug it in, and set it up to be the boot drive (VERY easy to do, this is literally child's play on a Mac).

You will be back here saying "I never believed just doing that could make such a difference in speed".

Do what I suggest, and you WILL be back here saying just that.

 

[ruslan120]

I’ve read this advice and have tried it with a Patriot 128GB and standard USB 3 enclosure for benchmarking purposes. For some reason, my experience was less than stellar, to the point of crashing apps.

Unless I did something wrong, I would recommend thunderbolt

Edit: To elaborate, everything was initially fast, but operations to transfer from another USB device to the boot drive (USB SSD) crashed Finder (2GB), multiple apps opening performed terribly, more than a few apps open - totally unresponsive machine

 

[ticotoo]

...I'd go so far as to say worst Mac we've owned ever...
A while back we bought new from Apple a late 2015, 27-inch, Retina 5K iMac. (3.2GHz i5, 16GB RAM, AMD Radeon R9) as our new family machine.
Now our first Mac was an LC475 (also know as 'that pizza box Mac...') so we've been through quite few different Macs over the years but I have never encountered one so bad as this machine.
Just insanely slow & laggy. Everything takes forever to happen with spinning beach balls on everything you try to do. Even right-clicking for a drop-down menu can take 5+seconds for it to appear.
I've run all of Apples diagnostics & keep it up-to-date but I'm guessing that this was just a total dog of a model.
First off - anyone else had any experience with this model?
And second... - anyone have any suggestions for further diagnostics or fixes?

What you are experiencing is a failing hard drive. The machine keeps reading the failing sectors over & over, before moving on. Sometimes the failing sectors are marked unusable & replaced with extra spare sectors which are limited in number. All this take a lot more time than just a good read. Easiest fix is get an external tb 2 disk docking station & a SSD(OWC docking station works fine). Clone your boot drive over to the ssd and set your startup disk to the external drive.