to commonly asked questions.

Why is my computer running so slow?

Continuing slowness/non responsiveness. Has gotten progressively worse over last year or so. Feels like hard drive failing.
I’ve done several hours of tech support w Apple over the last year, going to second and third level to no avail. Tried everything in the book
Finally took to genius bar in April. They said it passed all diagnostics and it’s not a hardware issue, so they wiped the drive, reformatted and reinstalled latest os
Was better for a few weeks then began slowing again (spinning beach ball etc)
Took back to Genius Bar last week. They kept it several days, ran tests which all passed. Wiped drive again and reinstalled previous os, high Sierra.
It’s still doing it. Maddening.

Last time you were here (couple years ago?) you speculated it could be one of my external drives causing the issue. I’ve tried unplugging them, using a test user acct off and on but can’t isolate the issue. Sometimes the Mac works fine and the next minute it freezes up.

First, dying hard drives are almost always the cause. I would say 90% of the time, these behaviors are indicators of a dying drive. It sounds like you have done all the right things to try to trouble shoot the issue. Creating a test user allowed you to eliminate any personal software. Good move. Disconnecting external hard drives, good move. Reinstalling the OS, usually a last resort but also a good move. Did you immediately restore the computer using Time Machine back to your old data again or use it as a fresh install of the OS only for a while? Could we attribute any developing slowness to the time machine restore? (if that was done)

Often, a dying drive doesn't do well when reformatting and reinstalling an entire OS so if the internal drive were dying it will often fail completely during a restore.

There are generally three reasons that cause a computer hardware to run slowly. (excluding internet speed, not a part of our discussion)

1. hardware failure
2. software problems
3. the OS/applications are far more advanced than the hardware and the hardware can't keep up with the processor requirements.

My gut guess here is that the reason you're having slowness issues is 1. hardware failure. Specifically the hard drive.

Number three can be eliminated by moving back to High Sierra and sounds like the problem still persisted. Number two can be determined usually by monitoring the information in "Activity Monitor". Sometimes (rarely) certain processes will run out of control therefore taking up 100% or more of the CPU. This can cause the speed issue. One such service known to do this is Photo Analysis; the process where your Photos app scans every pixel of every picture in your library and tags all items in that picture for future searching ability. You have eliminated this being the case by using a test user however and wiping the OS.

With the evidence presented here I would lean strongly toward hard drive failure. Is your computer a thin iMac or about 1" thick on the sides? Also, if you are using a thin iMac, what type of hard drive is installed? SATA drives in the thin iMac series are 2.5" spinning drives and known to fail. They're also not very fast drives. Fusion drives store some info on a flash portion and the rest on the spinning SATA drive. A SSD (solid state) drive is the fastest drive and almost never fail. This is due to the fact that SSD drives have no moving parts.

With all this being said, I may not be able to solve this problem in any other way other than recommending a new computer because it sounds like you've done many of the troubleshooting steps one would try to get to the problem. The only consistent factor left in all the testing is the hard drive. To find out more about the drive, you can go to the Black Apple (top left corner of the screen) and choose "About this Mac".

If you take a screenshot of the overview and storage areas that would help. Also if you could answer any of the questions included above.
This image is a theme.plist hack