new iMac odd and slow unless I reset PRAM every time

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

First and foremost you've installed some totally unnecessary and known to cause problems software on your iMac: Avast. There is no reason to ever run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac.  This user tip describes what you need to know and do to protect your Mac: Effective defenses against malware and other threats - Apple Community.  Uninstall it according to the developer's instructions.

You can check to see if you've removed all of the files by downloading and running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For example for Avast software you'd do the following search: 

1 - Name contains avast

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

 You should also uninstall Flash. It's not used any more in website, HTML5 is not the standard, so remove it and its System preference pane if installed. Adobe will be dropping it at the end of this year.

For 16 GB of RAM you have way too much swap

Available RAM: 4.71 GB

  Swap Used: 1.56 GB

You should limit the number of apps in your System/Users & Groups/Login Items page to on those that absolutely must be loaded at boot. Launch the others when needed and close when done. And reboot frequently, 2-3 times per week if you're one to just sleep your iMac.

Uninstall Avast per their directions , Restart then run etrecheck again to see what chaff it left.

On AVG & Avast…

https://x704.net/bbs/viewtopic.php?f=17&t=8803&p=115715&hilit=avg#p115715

https://discussions.apple.com/thread/251454104?answerId=252862395022#252862395022

The use of third party anti virus applications, cleaning applications, security applications and other “maintenance “ applications serve no purpose whatsoever. They offer no additional layer of security, maintenance or cleaning that Mac OS cannot do itself. If simply kept up to date and otherwise left alone Mac OS will run securely and for most users trouble free for many years.

When users beginning adding these types of third party applications Mac OS becomes, slow, unstable, and frequently acts like it is buggy. So if the OP is happy with an unstable system he by all means should leave Avast installed.

The other thing I commonly try is an SMC reset:

https://support.apple.com/en-us/HT201295

My concern over anti-virus software is based on all of the posts by users who had installed A/V, experienced poor performance afterward, removed it and fixed their problem. Time and time again that pops up. Most of the A/V software is not geared to the usual malware/adware that Macs are vulnerable to. The only app I'll use for that is the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

I'm not sure what John Gault has to do with it except he wrote a very comprehensive user tip on how to protect your Mac.