Slow iMac (Retina 4K, 21.5-inch, 2017) so slow and showing spinning beachballs?

Yes, it does not have the capacity to manage your data! The only method of making it work is to get an additional external HD and move large libraries (typically your photos, music and movie libraries) to the external HD. The goal is to get your storage down well below 480GB of storage, if you can get it 200-300GB that would be best. Otherwise I'd just pony up for the 1TB external SSD option or consider replacing the computer.

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Jack

Likely because it has a glacially slow 5400rpm HD, if it does that is its bottleneck. The only solution is to either replace the computer or buy an external SSD, then clone your HD to the SSD and use the SSD as your boot drive.

Check the System information app that is preinstalled to check to see about its HD.

As I guessed, the primary bottleneck is that the computer has a 5400 RPM HD which is incredibly slow to the point it slows down an otherwise good computer. In addition the computer has not been restarted in 20 days. Consider restarting about 1x per week.

To speed up the computer you will need to get an external SSD. To get extremely fast performance I'd recommend the https://eshop.mahttps://eshop.macsales.com/item/OWC/TB3ENVPSX01/lt... however if cost is an issue then the https://eshop.macsales.com/item/OWC/MSU3SSDT1.0/on and a https://eshop.macsales.com/item/OWC/MOTGPWR/ would be fine.

Once you have purchased and connected the external SSD use an app such as SuperDuper! or Carbon Copy Cloner and clone your internal HD to the new SSD. Then set the new SSD as your startup drive in System Preferences.

That should do it.

The are other options which include replacing the computer for a new model with an internal SSD. You can also have the internal HD replaced with a SSD however that would be very expensive as iMacs are difficult to work on.

Best of luck.

You can also read How to Setup and Use an External SSD as y… - Apple Community for the setup steps.

Jack

Whoops sorry on my part, I misread and thought your 1TB drive had 700+GB in use. If you have 700+ GB available then yes a 480GB SSD would be just fine.

Your welcome.