My computer is very slow. Ran EtreCheck and it says my HDD is slow. I have a fusion drive.

You can try Disk Utility Repair while in macOS recovery, but sadly it looks like part of the Fusion drive maybe failing.

see > Repair a storage device in Disk Utility on Mac - Apple Support

If you can not repair the drive with Disk Utility in macOS Recovery. Make sure that you have a current backup and schedule a service appointment at your local Apple Authorized Service Provider.

see > Genius Bar Reservation and Apple Support Options - Apple

The age of your iMac suggests that it probably would be more expensive to have the drive fixed or replaced by an AASP. Instead I believe you would be best served by getting an external SSD, cloning your boot drive to it and booting and running from it. When it's time to move on up to a new computer you can take the external SSD with you. This would save you considerable $$ for the labor costs of repair.

I suggest you contact OWC (MacSales.xom) Customer Support and see what they recommend as the best of their external SSDs for your particular iMac and budget. OWC is considered by many as the premier 3rd party hardware supplier for Apple computers.

Just some food for thought.

These are the data the triggered the "too slow"advisory:

Performance:

System Load: 2.02 (1 min ago) 2.40 (5 min ago) 1.88 (15 min ago)

Nominal I/O usage: 0.43 MB/s

File system: 74.57 seconds

Write speed: 38 MB/s

Read speed: 49 MB/s

Those speeds are horrible. For a Fusion drive with your specs, Write should be 500-1000MB/sec and Read should be 1200-2000MB.sec. Yours are half of what the badly under-spec mech-only drive in entry-level 21.5 iMacs of that vintage can do,

It is hard to tell from the report which component—the SSD or the mech drive—is failing. I doubt Apple will worki on a computer this old, so you may wish to consult a Mac-savvy independent service provider. There is not much more we can do here where we can neither see nor touch your computer.

The case was not meant to be opened so any work insode needs a pro unless you are skilled and adventuresome, and accept that any little slip-up when break the sale could destroy the computer. Were it mine and I wanted to keep in, I would have a pro replace the tiny SSD with a 1-2TB blade SSD

https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019

and simple disable the mech drive. Given the need for 2 hours labor chrage on top of new parts and the reseal kit. it could get pricy.

PS: Disconnect the Passport drive and see if anything gets better. They tend to be little troublemakers.

For reference, an external SSD as boot drive on my 2015 21.5" iMac Retina had read/write speeds in the 460 MBps range.

Made the 2015 iMac a joy to use. The internal HDD was reading at 93 MBps last I checked it.

Instead of paying an arm and leg to replace the internals with an expensive SSD upgrade. Another option for that year model iMac is to get and run from an external USB 3.0 SSD.

😉 Yup' I have a 1T PNY SATA SSD in a Sabrent USB 3.0 enclosure and the read/write speed is up around 480 MB/s.