iMac 21.5-inch 2017 running slow

I agree it is time to move on. You have the crippled educational/institutional iMac, made for bulk purchasers with low-demand computer needs.

It has a laptop-class 2.3gHz two-core processor. The consumer entry-level iMac that year, the Retina 4K 21.5-inch 2017) had a desktop-class 3.0gHz four-core processor. These benchmark scores from the MacTracker database† show the performance cost of a US$200 cost saving at purchase:

As others have identified, your hard drive, which could only transfer data at about 80 MB/sec when new, is now doing half that. The "timed out" file system test says the drive is seriously sick.

As Tims says the only speed-up option would be setting up an external USB-3 SSD drive as your boot volume. Even then that does not make the two-core processor any better or faster, and you cannot install any macOS higher than macOS 13 "Ventura," now off the "currently supported" list if macOS versions for many apps.

† — The MacTracker database is free in the Mac App Store. Nice resurce for comparing when shopping:

https://apps.apple.com/us/app/mactracker/id430255202?mt=12

Looks like you purchased the 21.5” price point model which only had a dual-core CPU (instead of a quad-core one), and a 1080p screen (instead of a Retina 4K screen.)

Your Mac has 8 GB of RAM, and there was plenty available at the time you ran the report. That dual-core CPU was not working hard, either.

Your Mac has a mechanical hard disk, and that is likely one source of slowness. Getting an external SSD, cloning your system onto it, and making it your startup drive could help with the time it takes to start up the Mac and to launch apps.

No matter what upgrades you make, that Mac cannot run any version of macOS higher than Ventura.

Re: “The hard drive, which was slow to begin with, is failing.”

I didn’t catch that. If the failure gets bad enough, the drive may start throwing errors that crash the Mac even if the OP gets an external SSD. Surgery to replace the internal drives on those 21.5” models is delicate and/or expensive. So this might be an argument in favor of a new Mac.

What type of work/play — what workflow — do you need the Mac for?

what budget you have for new hardware?

if the 2017 iMac and macOS are good — the external ssd is a good option — I would steer you away from usb and recommend Thunderbolt 4 enclosure

2017-2019 supports Thunderbolt 3 (tb4 compatible)

tb3 enclosure with NVMe ssd will more than double usb-c speed (on 2017-2019)

and tb3 will be some 30 times faster read/write than the original hdd

hit back if you want specific tb4 SSD items or more info..

As you've been told the hard drive is failing. Its speeds are ;

Write speed: 42 MB/s

Read speed: 42 MB/s

If you're thinking of getting a new Mac and budget is a concern consider the following: a 10 Core Mac Mini M4 with 16 GB RAM and 1 TB SSD ($1084) with a 32" LG 4k monitor ($331) from Amazon.com)  is $1415 which is $541 less than a similarly configured new 24" iMac ($1956)  re 07/2025. Keep your keyboard and mouse from your current Mac for use with the Mini.  I upgraded from a 2017 i9 iMac and the Mini runs circles around it. It's speeds leave the iMac in the dust.

I got the above Mini with a 32" LG 4K monitor w/speakers for only $60 more than the 27" (didn't know the Mini had a speaker).  There are many monitors available in 27" size on up for $100 to $500 depending on what features you want.  The monitor prices are before tariffs.

Just some food for thought.

The hard drive, which was slow to begin with, is failing.

I suppose you could get yourself an external SSD and use that instead of the internal drive, which should improve the performance. That's a short term solution, though. That machine can no longer run a supported version of macOS.

I would strongly recommend replacing it with a new machine. If that's not financially feasible right now, go for the external SSD. At least it the SSD would still be useful when you get a new Mac.