apple error 4SNS/1/40000000:TAOP-40.609

That code says Apple hardware Test found a problem with the ambient air temperature sensor. That is not a common cause of slowness. Most iMac owners' complaints of "slow" end up being a slow mechanical hard drive or unneeded third-party software like anti-virus or so-called "cleaning" apps.

To effectively help , we need to make sure there are no software conflicts before worrying about hardware issues. Too often software masks hardware issues or created false ones.

A good place to start looking for culprits is with a snapshot of your software configuration. Fortunately there is a safe, secure way to do that. 

We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck, available free from the Mac App Store here:

https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12

We can also see hard data about drive performance and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find):

When its report displays, click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.

⚠️ Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

NOTE: Changes in late 2018 to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Additional Text” window:

Thank you! I do not read French but, between Latin in university and a little "street" Spanish from a past career, I was able slog through the report. I had to use Google Translate only once!

First you MUST completely uninstall AVAST. It is not a nice program and totally unneeded on a Mac. Likely a big contributor to the slowness, such as the unidentified "other process." The company also has used some tricky marketing tactics in the past. See this ASC post:

Avast for Mac trickery. Beware. - Apple Community

Many VPNs will slow a Mac. Try testing with yours turned off.

You hard drive is running much slower that is should and Etrecheck flagged it. Before worrying with the disk, see if removing AVAST changes that number.

The runaway process is indicated but not identified here:

Autres opérations 121.00 % (?)

You may need to use Activity Monitor to find what was beyond Etrecheck's ability to identify. Please see this Apple article for how to do that:

See how apps affect Mac performance, battery runtime, temperature, and fan activity - Apple Support

Steps 1 and 2 are very important to getting useful information from that test. Also, the display is dynamic, changing every few seconds. You will need to watch the list change for 20-30 seconds and see what process is spending a lot of time at the top of the list. The nature of the display also does not make a screenshot helpful, so simply make a note of any suspicious processes' name(s) and tell us.

https://discussions.apple.com/thread/4157713?answerId=19098394022#19098394022

Disk failure... hopefully you have a current backup or clone.

OK, good luck my friend. :)

Great to hear on the new Mac, congrats! :)