iMac with sudden Performance Issues

Yes, I tried restarting the Mac. But still, the performance is poor.

I am worried that it is in fact the HDD. Since I have had issues with this drive before. Is a HDD in a failing state something the Apple Diagnostics Test does not pick up? Can this be repaired, or would the drive need to be replaced?

Here are the results of a second EtreCheck:

In Safe Mode the issue still persists. Per your advice I also installed the macOS Catalina 10.15.4 Combo Update, but yet again with no results. If anything – it made it slower. I tried running 10.15.4 in Safe Mode, so nothing was launched and ran another EtreCheck:

Everything from booting up, to loading Launchpad, to opening a folder. Everything freezes or takes ages.

Appreciating all the help – thanks!

Thank you both a ton for the help so far – I appreciate it.

Per the mentioned thread and advice I have fully removed both CleanMyMac X and Torrent-related apps from my Mac. I also checked the leftover files through Find Any File and deleted them as well. Unfortunately, after rebooting the Mac is still unbearably sluggish. Failing to simply load any app. Occasionally, even macOS itself seems to get clogged by removing desktop icons or the top bar becoming invisible.

Exactly. I'm puzzled.

Through Activity Monitor, at the time of checking, the 'remindd' process was taking up a consistent 100-110% of the CPU (and has been running since bootup, according to the CPU time). After force quitting the process the issue didn't go away though. Apart from that I regularly have 70-80% inactive CPU usage.. RAM is also in the green at all times, and power consumption doesn't look off either.

Thankyou all for your help. I have tried doing a full clean install of macOS and seeing if that solves the issue. From Disk Utility I right away noticed the Fusion Drive got splitted to SSD + HDD so I had to restitch them. Weirdly enough, it prompted me to install the original OS (OS X El Capitan). Upon doing so the installation went well. Then I restarted the Mac and sure enough was greeted with the following error:

'OS X can not be installed on your computer.'

POSIX-diagnosis: The operation couldn't be completed. Input/output error.

So then I separated the Fusion Drive manually into SSD and HDD.

Tried installing OS X on the pure SSD, and it worked wonderfully. Albeit older software, it ran as fast as the Fusion Drive did before all the issues started appearing. Ran an Entrecheck and it finished within 1:36 as 'Excellent Performance'.

So, I then tried the same for the HDD. Getting it to be reformatted on Disk Aid took very long. Installed OS X, but upon rebooting it wouldn't do anything. It would be stuck with a loading bar and the Apple-logo. Then I checked which drive was the boot drive, and the HDD would not even show up. Eventually resulting in: 'An error has occurred during installation.'

Disappointed that I can not check the speed on the pure HDD and compare it to the SSD. Nonetheless I am now convinced that there is enough evidence to claim it is not the software. ;)

Am I right to guess the HDD is the issue, although it shows up as 'fine' through the Diagnostic test?

Also, if I were to replace the old HDD with a new SSD, what should I do with the old NVMe SSD of the Fusion Drive?

Thanks both for your advice and troubleshooting steps along the way.

I have now fully replaced the old failing HDD with a new SSD, and restored through Time Machine.

The machine is now shining once again, hopefully for the years to come.