2015 iMac super slow (Retina 4K, 21.5-inch, late 2015)

Newer Mac OSes use close to 8 GBs of RAM, now.

macOS High Sierra is probably using a good chunk of your available RAM!

You cannot install more RAM into this year and model iMac as it is soldered onto the logic board in the 21 inch screen models

Here's the BIG issues with all, but the topped out, 2013-2015, slimline, 21 inch screen iMac.

The 2013-2015 slim profile 21 inch screen iMacs that Apple had offered are, IMSO, "downgrades" from the previous 2009 thru 2011 21 inch iMacs models.

The 2013-2015 slim profile, "slimline", 21 inch screen model iMac are, basically, sealed up, glued up, basic computing appliances, now, with only the "top of the line" with all the top options (top i7 CPU, 16 GBs of RAM, 256 or 512 SSD) being the only viable option to use as a standard to medium professional grade, work related, iMac with just enough power for professional and creative use.

There is nothing in the 2013-2015, 21 inch screen iMacs the is user upgradeable OR serviceable, any longer.

Slower laptop 5400 RPM hard drive/fusion drives, limited RAM (the 2010, 2011 i5 CPU models and higher could take up to 32 GBs of RAM).

These 2012-2015 slimline 21 inch screen iMacs are limited to just 16 GBs of RAM.

Slower 5400 RPM laptop standard/style hard drives are 33% slower than the equivalent storage size in a "desktop standard" 7200 RPM hard drive.

This means 33% slower read/writes to the drive, in addition to other system delays.

The slower 5400 RPM laptop grade hard drive IS THE biggest bottleneck in the 2013-2015, 21 inch screen iMac models!

Also, like laptop models, these older 2013-2015, 21 inch screen iMacs no longer use dedicated, discreet and independent GPUs.

These older slimline 21 inch screen iMacs use the "so-called" integrated (soldered) Intel Iris ”Pro" GPUs, now!

Meaning that in any of the newer 21 inch screen model iMacs, if you get one with the max 16 GBs of RAM, a little bit to quite a bit of that installed RAM is being used for the GPU's VRAM!

Meaning that the GPUs in these newer 21 inch screen model iMacs is stealing away physical RAM, for use by the GPU, for the GPU's VRAM, that could be needed for any other possible RAM intensive tasks depending on the apps being used or what combination of apps is being used together.

The GPU will automatically steal whatever the amount of physical RAM the the GPU needs for any type GPU related task.

This would not be as big an issue if you could install more than 16 GBs (which there is no way to do this in these 2013-2015 , 21 inch screen iMacs as Apple has soldered every single component to the logic board).

The 2012-2015 baseline 21 inch screen iMac models only come with 8 GBs of RAM standard.

New versions of the Mac OS need a base MINIMUM RAM of 8 GBs, now.

So, 8 GBs of RAM is really not enough RAM to run both the OS AND any additional applications.

Especially the type of applications that are SERIOUS hardware resource intensive!

To get a better performing iMac, now, you would have to spend the money for all of the top end options for the new 21 inch screen model iMacs to ensure better longevity and "futureproofing"

OR

You would really have to graduate up to the more expensive 27 inch screen iMacs that still have 7200 RPM hard drives/fusion drives, independent GPUs and user installable/upgradeable RAM.

I am NOT a big fan of these 2013-2015, 21 inch screen iMac models, at all!

Other than a top shelf version 21 inch screen model, the other 21 inch screen model options are like a Mac Mini with a screen, now, or like a laptop/desktop hybrid, sealed up appliance.

To greatly improve the speed and performance of your 2015, 21 inch screen iMac, you are going to have to spend some significant more cash for this to happen.

My solution is to go the less difficult plug and play (somewhat) route.

Purchase an externally enclosed USB 3.0 or Thunderbolt connected SSD.

Format the SSD for Mac OS extended (journaled) with GUID partition scheme and then purchase data cloning software, like Bombich Software's CarbonCopyCloner or Shirt Pocket's SuperDuper to make a bootable clone of your current system to the SSD and boot and run your iMac, exclusively from the externally connected and enclosed SSD.

Your iMac will run A WHOLE LOT FASTER from an externally connected and enclosed SSD!

You can either opt to use your internal iMac's hard drive to just store data OR purchase, yet, another external USB 3.0 or Thunderbolt connected 7200 RPM mechanically spinning hard drive to use as your primary data storage and data backup.

If you are from the U.S., here is a good Mac/Apple source for external SSDs and standard 7200 RPM hard drives here.

https://eshop.macsales.com/shop/external-storage

You can call them to speak with a sales/technical rep or contact by email

Your RAM and GPU are still problematic in these 2013-2015, 21 inch screen iMacs, though.

There is nothing to be done with that!

Good Luck to You!

In the meantime limit how many apps you run at one time. I have two Mac laptops with 8GB of RAM and have no memory issues with High Sierra.