imac 2012 so slow its almost unusable!

+1 on the failing hard drive and its causing slow-downs. However, even were the drive up to its max potential transfer rate of about 80MB/sec, the computer would still "feel" slow.

This is an old computer that is probably under-powered for Sibelius and cannot be easily opened for internal upgrades or service. Fortunately, you have a cost-effective external option that avoids expensive internal servicing.

You can get an empty USB3 external drive enclosure (preferable with its own power supply) for US$50-70. Into it goes a solid-state drive (SSD) rated at SATA 6GB/sec that will cost you about $135 for a 1TB model or $75 for a 500GB. Clone your existing drive to the external one with either SuperDuper or Carbon Copy Cloner.

System Preferences > Startup Disk lets you set the external SSD as your boot volume. Why is this better? That setup should allow data rates of around 400MB/sec compared to your current wounded drive which whe new could not do faster than 80MB/sec.

With the new faster drive, you have avoided having a professional open and gut the computer to fix the wonky drive.

Spend $200 on parts for a 2012 computer? Wouldn't make sense if it only worked on that old iMac, but you can move the SSD assembly to act as a fast external drive on your next computer, Mac or Windows.

You may well need a newer computer seeing that your's is 8 years old. It probably has a HDD drive which is slow to begin with. You will need to post a lot more info for anyone here to be of help to you so get a copy of etrecheck from the Apple Store , run it and post the results here.

In the meantime, if you have any antivirus or so called cleaning apps installed, get rid of them by using uninstall directions from the original developers and try running without them. They are totally useless on a Mac and will definitely slow things down and may otherwise screw up your system. Then restart your Mac and try it again.

Do make certain you have good backups.

Failing hard drive - This machine has a hard drive that appears to be failing.

It isn't always failing when you see that message, could be etrecheck thinks something is wrong with it just due to so little free RAM & so much swapping going on, slowing the already slow drive like crazy.

Delete these files & restart...

~/Library/LaunchAgents/com.amazon.music.plist

/Library/LaunchAgents/com.kodak.BonjourAgent.plist

~ tilde indicates hidden Local Library

Apple hid the Users' Library folders...

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.

Safe Boot, (holding Shift key down at startup).

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, do a regular boot.

Good adsvice si far.

I think we can figure it out if you can submit an etrecheck report...

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The important part is...

https://pastebin.com/embed_js/KuvnghqA

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...