What's the best macOS for a 2015 iMac?

RE: "Am I running a OS that is too advanced for my iMac?"

I'm sure that Monterey loves RAM and fast startup disks. So let's do a quick review:

iMac (Retina 5K, 27-inch, Late 2015) - Technical Specifications

That machine would have shipped with at least 8 GB of RAM. Since it is a 27" iMac, the RAM is user-upgradable. The official limit is 32 GB, but the actual limit is 64 GB (as per MacTracker, Other World Computing, and Crucial).

If you think you don't have enough memory, you might want to use the Memory Pressure graph in Activity Monitor (while doing the things you normally do) to see if it shows yellows and/or reds. But even if you have 8 GB of RAM now, that's enough that a simple Finder search in Monterey should not be causing the hangs you report. Looks to me like something is going on other than a "not enough RAM" problem.

The machine would have shipped with one of these drives:

• a 1 or 3 TB 7200rpm SATA HDD,
• a 1, 2, or 3 TB Fusion Drive, or
• 256 GB, 512 GB, or 1 TB of flash storage

If your drive is a pure hard drive, an external USB 3 SSD startup drive would definitely help you with overall performance. If it is a 1 TB Fusion Drive, I also think an external USB 3 SSD startup drive might be in order. (The 1 TB Fusion Drives for this model year included only a miserly 24 GB of flash storage.)

If you're lucky enough to have flash storage, it's faster than anything you could attach to the USB 3 ports. As for a 2 TB or 3 TB Fusion Drive, those included 128 GB of flash storage, which might be enough for their performance to hold up well even under Monterey.

Another thing to check: S.M.A.R.T. status.

If you go into About This Mac > System Report… and look under the NVMExpress and SATA sections, do you see any lines that say "S.M.A.R.T. status: Failing"?

Hard drives will sometimes give advanced warning of failure in this way. A failing hard drive might be throwing up errors in such a way that one moment, everything's running along normally; the next, everything just hangs.

Monterey should not really tax your iMac any more than Catalina or Big Sur.

It could be anything from a failing hard drive, to some third-party software that is messing things up.

To get a better idea of what is happening to the iMac without going back and forth playing 20 questions,

get and run EtreCheckPro from > https://etrecheck.com/en/index.html

Then if you need help decifering the iMac's EtreCheckPro report? Please post the report using the Add Text feature,

as per > How to use the Add Text Feature When Post… - Apple Community

Wow besides setting a new record and the slow Write speed 137 MB/s of your drive, you have a lot of third-party software processes running on that iMac.

Try starting up in Safe Mode (it could take awhile) and see how it runs for just the basic macOS tasks.

Use safe mode on your Mac - Apple Support

If that improves the performance, you need to start by pruning the unused third-party software.

If that does not improve the performance, then you most likely have a failing hard drive.

1) Do Safe Mode first.

Most any inexpensive generic or pc USB keyboard will work with your iMac.

2) Reinstall the macOS over the top, instead of wiping and reinstalling from scratch.

see > How to reinstall macOS - Apple Support

Hi,

Monterey is stable macOS, so I think Monterey is better macOS for your iMac even if you face with the troubles which caused by hardware. Try to run Apple Diagnostics if it reports hardware related error.

By the way, how much memory do you install on your iMac?

Delete these files & restart...

/Library/LaunchAgents/com.adobe.ccxprocess.plist

/Library/LaunchAgents/com.parallels.desktop.launch.plist

/Library/LaunchDaemons/com.parallels.desktop.launchdaemon.plist

Whew, as John Balt says, & a record by maybe 10 fold!

Runtime: 377:17

That is an all time record.

Please determine if the same problems occur in "Safe Mode": Use safe mode on your Mac - Apple Support

Monterey runs beautifully in my 2015 21.5" iMac retina.

8GB RAM; 1TB Samsung EVO 860 SSD

Try right clicking on it & choosing Open.