is there a way to diagnose performance problem with Mac?

I'm surprised no one noticed the Protect Works anti-virus software that is installed, although it was not exactly listed in the usual location in the report. You definitely want to uninstall Protect Works anti-virus by following the developers instructions. Anti-virus apps, cleaning/optimizer apps, and third party security software are not needed on a Mac and usually causes more problems than they solve because they interfere with the normal operation of macOS. See this excellent article written by a respected forum contributor:

Effective defenses against malware and other threats - Apple Community

The SSD write speed is extremely poor, but that could be related to the anti-virus software. I would recommend running DriveDx (free trial period) and posting the complete text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar.

After uninstalling all the anti-virus software plus all the other items mentioned by the other contributors, then you should run & post another EtreCheck report so we can see if it has made any difference.

+1 please post an EtreCheck report.

The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

This Fusion drive issue hugely affects "slow":

Performance:

System Load: 1.66 (1 min ago) 2.04 (5 min ago) 2.34 (15 min ago)

Nominal I/O usage: 6.38 MB/s

File system: 18.53 seconds

Write speed: 211 MB/s ⚠️

Read speed: 1747 MB/s

The Read speeds, at ~1700MB/sec are appropriate to your Apple Fusions (hybrid) drive and in the healthy range.

The Write speeds are NOT. They are only 1/4 to 1/3 of what they should be in that computer. A healthy Fusion drive in your iMac model will post Writes of 600-900MB/sec.

Was the computer logged into your account or your husband's at the time you ran EtreCheck? I think we need ot see each one separately.

Have him check and remove login items in his user account, to see if that helps.

see > Open items automatically when you log in on Mac - Apple Support

Not that it's likely a big problem, but itshould go.

Delete this file & restart...

~/Library/LaunchAgents/com.adobe.ARM.***.plist

~ tilde indicates your home folder

Apple hid the Users' Library folders...

Method 1:

1 From the Finder, select the Go menu at top of the screen, and choose Go to Folder.

2 In the window that opens, enter ~/Library, and click Go.

You can try downloading and running an etrecheck report and posting it here:

https://www.etrecheck.com/en/index.html

The best thing I can offer is to pay attention to when something happens that seems to be not right. You might also keep notes. Look for a pattern. Is it after something was done? Has anything changed which might be related?

The report looks okay. Please download the free version of FindAnyFile and have it run and remove anything with the following in it:

• Java
• Oracle

Java is very old and antiquated and prone to downloading malware so it should be removed. To migrated from the old Mac to the new one please do the following:

When you buy a new Mac the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes  and follow the on-screen prompts and it's done! It is THAT simple.

If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts. 

The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use. 

Good catch, etrecheck threw me off he scent...

Antivirus software: Apple