Painfully slow iMac (Retina 4K, 21.5 inch, 2017) - etrecheck report included

To clone the drive you can use either SuperDuper! or Carbon Copy Cloner. Also as you will be buying this used, please read and execute the steps in What to do before selling or giving away your Mac. Once done, you will have a fresh iMac. Then when you get the external SSD you can clone and start over new.

+1

The reason your iMac is slow is due to the 5400-rpm hard drive. While this hard drive found in your Mac is perfectly suited to light tasks, such as email, web browsing, etc., for more advanced tasks, and for users who want the maximum performance from their Mac, this hard drive does have its limitations. Luckily, you can use an external SSD as your startup disk to run macOS and all your data from that. It will make your Mac 10 to 30 times faster for storage, and in turn, make your entire Mac system much faster. For more info, instructions, and what external SSD to buy, please see: How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac - Apple Community.

Another option is to replace this Mac with a new Apple Silicon Mac: Mac - Apple. If you go this route, it is generally recommended that you purchase 16GB of Memory with the largest SSD that you can afford. You can also use Apple Trade-In to get credit towards a new Mac: Apple Trade In - Apple.

Jack

If the price is $300 or less it may be worth buying and then to speed up performance add a OWC Mercury On-The-Go Pro 1TB SSD Storage Solution and then clone the internal HD to the new SSD and then be ready for good experience. Simply set the SSD as the startup drive after the cloning process and you are good to go.

If the price is >$300 save your money and buy a new 24" M1 based iMac with 16GB of RAM and the largest capacity SSD you can afford.

The "tale of the tape" for the hard drive is here:

Performance:

System Load: 1.34 (1 min ago) 1.38 (5 min ago) 1.47 (15 min ago)

Nominal I/O speed: 11.29 MB/s

File system: 81.00 seconds

Write speed: 22 MB/s

Read speed: 20 MB/s

The drive, when healthy, was never speedy but should do closer to this, an EtreCheck report from an iMac with the same HDD specs:

Performance:

    System Load: 1.85 (1 min ago) 2.19 (5 min ago) 2.40 (15 min ago)

    Nominal I/O speed: 0.00 MB/s

    File system: 36.90 seconds

    Write speed: 80 MB/s

    Read speed: 82 MB/s

The external SSD solution rkaufmann87 mentions would bump the write/read scores to around 400MB/sec, but you must order a USB3 enclosure with an SATA 6GB SSD inside to achieve to get full benefit. 400 MB/sec feels very fast in use.

I see the computer is currently running Creative Cloud. It is far under-spec for Adobe Pro apps in my opinion.