2017 iMac running slowly. Need help to clean up.

Before you do ANYTHING else and I mean ANYTHING please backup the computer or you will likely lose 100% of all of your data!!!!!!

Your computer is 7 years old and likely has a failing hard drive. It is also fitted with a glacially slow 5400 RPM HD. If you want to keep the computer (I'd replace it myself) then get yourself an external SSD and then use How to setup an external SSD as your startup disk to use the SSD as your startup drive.

You can confirm the HD is failing by downloading DriveDX (DO THIS AFTER BACKING UP!!!!!) and run a report on the internal HD and if any errors appear that only confirms what that my hunch of a failing HD is correct. However, as I have stated 3x now backup first! If you don't know how then click Back up your Mac with Time Machine

First you should be aware that you purchased the slowest iMac that Apple offered at the time of purchase: 8 GB RAM and a 5400 rpm hard drive.

It appears that your Fusion drive may be failing with it's extremely low read and write speeds:

Write speed: 55 MB/s

Read speed: 60 MB/s

A possible fix for this is in the last two paragraphs of my reply.

I don't see any of the usual suspects in the report but noticed that you are using Google Chrome as your browser. It's know to be a serious CPU how as demonstrated by the CPU and Memory Snapshots in the report. If you don't like Safari give Firefox or Brave a try. Both are less demanding on the system resources than Chrome.

Because just deleting the Chrome app leaves a lot of flotsam behind to get rid of the many supporting files that Chrome installs I would use AppCleaner to remove the app and its supporting files. But heed the warning below:

WARNING: If you use AppCleaner on an app that you have other apps from the same developer, like Adobe, you must be extremely careful checking all checkboxes and deleting.  Some for those files may support other apps from the same developer and deleting them can mess them up.  Adobe apps is a primary example. I know from experience.  For singular apps from a developer it's safe.

To speed up the general performance, boot time, app and file opening and saving time you can get an external SSD, clone your system to it with Carbon Copy Cloner and boot and run from the SSD.

I suggest you contact Customer Support at OWC (MacSales.com) and get their recommendation for the optimal SSD for your particular iMac, work requirements and budget. OWC is considered the premier 3rd party hardware supplier for Macs.

I don’t see any indication of a hard disk drive failure, but this is a fairly old iMac with a hard disk.

Some background on this iMac: Why is my hard disk drive iMac so slow? - Apple Community

That article explains why this iMac is slow, and links to another article describing how to migrate to and use an external SSD to boost its performance pending replacement. (This approach was mentioned in a reply above, too.)

Backups are about the only way to mark your data as being actually valuable. Otherwise, you’re one drop, one dunk, one ding, one loss or theft, one command error, or one hardware failure away from data loss.

You do not have Fusion drive so those drive speeds are only a little slow, but still too slow for newer OS versions.

The elephant in the room is that this is the crippled educational/institutional iMac made for bulk buyers looking for a price break.

Hardware Information:

iMac (21.5-inch, 2017)

Status: Supported

iMac Model: iMac18,1

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

It has a slow 2.3 ghz, two-core laptop-class processor instead of the regular consumer "4K"model with a desktop-class 4-core running at a minimum 3.0 ghz. It also has slower integrated graphics instead of discrete graphics, and half the display pixel density of the regular consumer model (iMac 4K).

For a saving of about 18% in purchase price you got a huge penalty in performance. These are GeekBench scores from the Mactracker database for your model and the next model up, with yours in the upper position:

Adding RAM is close to impossible in that model and won't change the slow, entry-level hard drive and/or the slow processor. Adding RAM requires a nearly complete gutting of the computer. Some Apple Authorized Service Providers will not add RAM to any 21.5-inch iMacs made between 2012 and 2019 dues the high labor costs and risk of computer damage.

What WILL help is the external SSD solution our respected Toad mentions. Nothing else that is practice will change things, other than maybe giving Chrome the boot. It can be a real resource hog on entry-level equipment, as shown in your report:

Top Processes Snapshot by Memory:

Process (count) RAM usage (Source - Location)

Google Chrome Helper (Renderer) (16) 1.78 GB (Google LLC)

EtreCheckPro 668 MB (Etresoft, Inc.)

corespotlightd 318 MB (Apple)

Google Chrome 306 MB (Google LLC)

kernel_task 224 MB (Apple)

It is using 25+% of your meager amount of RAM.

You have not restarted in About 6 days.

That iMac has a really slow 5400rpm HDD.

You are running Google Chrome and MS Office.

First off it might help to restart or shutdown and startup everyday.

After that you should try using Safari instead of the resource hog Google Chrome.

Then you should try and quit or stream-line your use of MS Office in conjunction with other Apps.

In the end there are two option to improve performance.

1. Replace that iMac with one that has an SSD.
2. Run that iMac from an external SSD.

Thanks to Jack-19 for the follow user tip

Use an external SSD as your startup disk … - Apple Community

Delete this file & restart...

~/Library/LaunchAgents/com.hp.devicemonitor.plistupport/ShadowsocksX-NG/privoxy

~ 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.

BDAqua wrote:

…

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.

3: Use the Command-Shift-. toggle in Finder to show or to hide hidden files and folders

Most of the Fusion drives I recall seeing in Etrecheck reports were in the 400-500 range for speed. 50 and 60 are the slowest I've ever seen.