Why is my new (2017 model) Mac so very very very slow? Is it faulty?

A 4 year old computer is hardly new. I suspect you bought a base model 21.5 2017 iMac which has a glacially slow HD. Those were only designed to do emails and very basic Internet surfing and NOT run professional grade apps such as Adobe apps. Return the computer if you can and buy something with a minimum of 16 GB of RAM and the largest SSD you can afford. If you cannot return it then do the following.

Buy an external SSD and migrate the internal HD tithe SSD. The use the SSD as your startup drive. The lesson to learn. Is to NOT buy a computer based on its price but to purchase one that has the specifications to manage your needs.

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.

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.

Copy the report

and use the Additional Text button to include the report in your reply.

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

Welcome to Apple Support Communities.

Chances are, it is a slow hard drive. But, to check, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

-Jack

Like Allan said, your iMac is the base-line model.

What I would do to help performance quite a bit is to startup the Mac from an external SSD, since the main reason your iMac is very slow is due to the 5400 rpm hard drive. Very slow. If you use an external SSD and run macOS from that, it will make your Mac 10 to 80 times faster for storage, depending on what external SSD you buy. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

-Jack

My guess is an external SSD to boot from would help.

If you'll post the Etrecheck report maybe we can find some things to help improve performance.

I am sorry you are having the trouble I am having. I bought my 2017 iMac in 2017 and it worked fine until I upgraded the MacOS to Catalina. I found a thread on the Communities Board recently about the slowness someone was having after an upgrade to Big Sur. The fellow had a fusion drive like I do and the person who replied said that the fusion drive might be the problem. You have Big Sur I see so If you have a fusion drive that might explain your slowness. Just a thought. More knowledgable folks might be able to shed more light on this. As for my situation, I have decided not to upgrade to Big Sur until I find out further information on how to overcome the slowness issue.

fi99 wrote:

Moving ahead , am wanting to increase the RAM but now it looks as if this is not possible on a 21.5" ?

Just to add, it is possible to have an AASP add RAM. You just have to find one willing to do so. It's not something you should attempt yourself.

FWIW, there was a non-Retina 21.5" 2017 iMac (Model iMac18,1) being sold up until a few months ago. It was the least expensive option available. It doesn't sound like it was your best option.

Hardware Information:

iMac (21.5-inch, 2017)

iMac Model: iMac18,1

2.3 GHz Dual-Core Intel Core i5 (i5-7360U) CPU: 2-core

You bought the crippled "Edu-iMac," intended for limited light use in an institutional setting. It is grossly under-spec for effectively using Adobe CS/CC.

Instead of a fast quad-core processor like the next model up, this has a slow laptop dual-core processor. It can never be fast.

It has has a slow mechanical hard drive, about the only thing you can address without trading the computer in for one with specs appropriate to Adobe pro apps. You can attach a USB-3 external drive containing a 6GB/sec solid state drive (SSD) and see a significant increase in launch and app-load times. However it will not render big CS/CC projects any faster.

Here are benchmarks from the MacTracker database for yours (top) and the next iMac model up that starts a US$200 more than yours:

For a roughly 18 percent increase in purchase price, you could have a minimum 58 percent increase in processing power.