Why is my iMac running heavy CPU usage, despite there being no apps using this CPU?

[overground]

I was using Adobe photoshop pretty heavily on my computer, and eventually this heavy use started to work up the fan in my computer. It was concerning because my iMac started to get unusually noisy, almost like a vacuum cleaner or a jet engine, and it went on for some time, even after I started closing applications. Also, the computer seemed to be noticeably slower to respond to my actions. I looked in the activity monitor and this is what I saw:

https://imgur.com/nQMEUpp

The noise did not stop, so eventually I shut down the computer, and it is now back to normal, although I haven't put a heavy workload on it since this occurred. Why am I'm having trouble with my computer? Is this sort of behavior unnatural, and should I return it while I'm still in my 30 day warranty? This was the first time it got so noisy, within the first two weeks of owning it.
What can I do to prevent this from happening again?

Thanks for any help given.

 

[sublunar]

For Photoshop you probably showed us the wrong pane. The Memory and Disk tabs would be more useful.

Which version of macOS is that? Is it fully patched?
How much RAM are you running in your iMac?
Is it fully SSD, Fusion Drive or HDD?
Whatever your storage is, is it almost full? (ie over 90% full?)
How complex are the 'heavy' photoshop files you are editing? What's the file size of the file that you were working on?
Why were all those other apps running at the same time? Even if no files were open they do take up working RAM (which is why the Memory tab is useful).

 

[MacManiac76]

You need to also change it to Show All Processes.

 

[overground]

For Photoshop you probably showed us the wrong pane. The Memory and Disk tabs would be more useful.

Which version of macOS is that? Is it fully patched?
How much RAM are you running in your iMac?
Is it fully SSD, Fusion Drive or HDD?
Whatever your storage is, is it almost full? (ie over 90% full?)
How complex are the 'heavy' photoshop files you are editing? What's the file size of the file that you were working on?
Why were all those other apps running at the same time? Even if no files were open they do take up working RAM (which is why the Memory tab is useful).

Thanks for posting. If this situation comes up again, I'll post once more, addressing each question you brought up.

 

[Yester5]

My 5K iMac will run perfectly for a few months, then the fan will kick on, the OS will bog down, and Activity Monitor shows anywhere from 12 to 30% CPU available at idle. Apple Genius Bar ran diagnostics as did Apple tech support engineers and they all say it's not a hardware issue, but software related. I erased the hard drive, reinstalled the OS (started happening back in High Sierra, now up through Catalina), restored from Time Machine backups, you name it. It always comes back...….sucks BIG TIME
I have a 2014 5K iMac (1st gen)
3TB Fusion drive (only 1.5TB used)
32 GB RAM