IMAC (2017 8GB) is running out of RAM while macbook pro (2013 8GB) is not

You didn't buy a top-notch iMac... you bought the entry-level 4K.

The problem is almost certainly the HDD. The entry 4K models uses a slow 2.5" 5400rpm laptop HDD that is exactly fast enough for MacOs/Safari plus Apple iWork applications. El Capitian and newer leave so little margin on those drives beyond the OS's own use that it is dreadfully easy for a couple of 3rd party applications (and their associated background updaters) to bottleneck on drive I/O.

It feels like four out of five macs trying to run El Capitian or later with with a 5400rpm HDD, 8GB Ram, and any 2+ of [Adobe, Microsoft, Google, File Syncing Apps, Antivirus/cleaners] have beachballs.

If you are within the return window I'd suggest taking it back to the store. Otherwise I'd suggest reinstalling MacOS onto an external USB-C SSD and booting from that.

Virtual Memory Information:

Physical RAM: 8 GB

Free RAM: 612 MB

Used RAM: 6.33 GB

Cached files: 1.07 GB

Available RAM: 1.67 GB

Swap Used: 1.17 GB

I find it very odd that swaps are so high after less than a day since a restart. That could be CMM at working its evil.

Top Processes Snapshot by CPU:

Process (count) CPU (Source - Location)

plugin-container (5) 34.76 % (Mozilla Corporation)

Other processes 20.63 % (?)

firefox 13.48 % (Mozilla Corporation)

EtreCheck 4.96 % (App Store)

Microsoft Word 0.65 % (Microsoft Corporation)

FireFox/Mozilla processes are certainly using a lot of CPU cycles (48%) considering they should be in the background while EtreCheck is running. Try using this Apple article to see if any other background processes are contributing:

See how apps affect Mac performance, battery runtime, temperature, and fan activity - Apple Support

⚠️ NOTE: Step One in those instructions is critical to getting useful results

Also, Activity Monitor is a dynamic utilties whose display changes every few seconds. No need to try a screen shot, but watch the changes for a minute or so and see if any processes with high CPU usage bubble to the top. Note the process name and tell us what it is.

It appears you may have two "versions" of Flash installed which "appear" to have the same version number although the second one's name is different. It is legit or malware? I haven't had Flash installed on my system in over 5 years so I'm unsure.

8GB of RAM is definitely the bare minimum for a system these days for basic use, but 16GB is recommended for any serious work. I have 16GB and I still run out of RAM when I have multiple browsers open with multiple windows/tabs each. Browsers today chew up a lot of memory due to the security sandbox, but also due to all the crap websites push out in the form of advertisements & tracking.

Whenever you run low on memory and macOS compresses it or swaps it to the hard drive, you will get a performance hit. You will really notice it when it swaps memory to & from the slow hard drive. A hard drive usually only transfers data at an average rate of 100MB/s (could be a bit faster or even slower) while your SSD in the laptop can transfer data up to 450MB/s. That is 4x the performance right there.

While EtreCheck shows Ok speeds on your hard drive, you could try checking the drive's health using DriveDX. If DriveDX reports the drive as healthy, then post the report here as it may contain clues which may not be considered a failing state by the app.

Okay, perhaps someone else will have some ideas that I have not thought of. Also, CMM is still installed. Consider doing an erase and install and starting over.

Good luck.

Another thing I forgot to mention which I noticed in the EtreCheck report: Time Machine has 24 local snapshots. I don't use Time Machine and I have only a basic knowledge of how APFS snapshots work since I haven't read much about them, but having 24 local snapshots might slow a system down especially on a system using a 5,400 RPM Hard Drive.

To expand padams35 solution, you can also upgrade the HDD to an SSD - which may be cheaper than even returning it and getting a better iMac (Even through an authorized retailer).

All Thunderbolt-3 ports are also USB-C, but not all USB-C are TB3.

USB 3.0 is sufficient.

USB 3.1 (connected to the Thunderbolt-3 / USB-C port) is just a little extra overkill, so if you have an empty thunderbolt port you might as well use it for the SSD.

Using an actual Thunderbolt NVMe SSD is complete unnecessary overkill for 99% of users.

First, uninstall CleanMyMac per the developers instrucitons, most experienced users consider CMM as malware. I would recommend doing the following so we can get an idea of what may be installed on the computer and what is using your RAM as 8 GB is considered a bare minimum.

Please navigate to the Mac App Store and download the free version of EtreCheck OR please click www.EtreCheck.com where you will find the app. Once you downloaded the free app and created the report please post it as your reply to this message. In order to post it, please use the following link for more detailed instructions on how to correctly include the results: 

https://discussions.apple.com/docs/DOC-250000211

We can then look for obvious issues that may be causing your problem

Your report indicates CMM is still installed, if you have already uninstalled it please run another EtreCheck report and look to see if any evidence of it is left. If you see any evidence it's still installed, hence why we call it malware!

Your computer seems to be operating fine, the possible trouble is you may have purchased the wrong specifications to do the work you want. Your iMac is a base model with only 8GB of RAM, this is considered the bare minimum amount of RAM to run Mac OS. It also has an extremely slow 5400 RPM HD. These machines are great for surfing the Internet, doing e-mail and very basic functions.

I see you have some Adobe apps installed, these tend to use a lot of RAM which means your computer is likely doing RAM swapping which means it is basically borrowing RAM from running the OS to run applications like Adobe and MS Office.

In my opinion (you know what they say about those!) you should have gotten an iMac with a minimum of 16GB of RAM and a SSD for storage. Had you done so your performance problems likely would not be happening.

What you will need to do if you want to keep this iMac is to learn to use the resources you have. The best method is to manage your RAM usage. Mac OS has a utility called Activity Monitor built into it. You can find it in Applications - Utilities. The following link will give you instructions on how to manage your resources including the RAM. https://support.apple.com/en-us/HT201464#memory

Good luck!

USB 3.1 <insert brand of choice> SSD was my recommendation.

If you don't have a favorite brand then: https://www.macworld.co.uk/feature/mac/best-ssd-mac-3600870/

For size I'd tentatively suggest 50-200% more storage than you currently use today. You currently have 330GB on the startup disk, so something between 500GB to 1TB. How much more data do you personally expect to store? Would you rather err large today or add an extra drive if/when needed? Do you plan to store documents/libraries on the old internal HDD or keep everything on the SSD? (These are rhetorical questions. You probably know your use better then us.)