Thanks you!
It's something with your Fusion drive. Apple Fusion consists of a small solid-state drive (SSD) linked by software to a larger mechanical hard drive. Normally fast, yours has a serious issue:
Performance:
System Load: 1.72 (1 min ago) 2.48 (5 min ago) 2.15 (15 min ago) Nominal I/O usage: 0.00 MB/s
File system: 20.22 seconds
Write speed: 237 MB/s ⚠️
Read speed: 2290 MB/s
For your iMac model, Writes should be 600-900MB/sec.
Although I don't think you have the common problem where the two drives forget how to talk to each other, you can read this Apple article:
How to fix a split Fusion Drive - Apple Support
and only do the quick test shown under "Before you Begin" to determine if your Fusion drive is still a Fusion drive.
If that show it is still a Fusion, that suggests one of the drives—likely the mech drive—is failing and you need to seek Apple help. Your iMac is still a supported model for service.
Make sure you have a full local backup—NOW!
I do not see any glaring software issues. I do have a bit of general concern with BackBlaze showing up in other slow iMac reports, but your slowness is clearly a hardware issue, your drive.
