iMac occasionally gets very sluggish. Memory Pressure is high and its hard to use computer. Why is this?
A dying a hard drive will actually cause this symptom as the system &quot;waits&quot; for read/write operations to complete.  It'll eventually get there, but often times, it will crash.
From the comments, you've confirmed this with the SMART status saying &quot;Failed&quot; which means that drive literally has one foot in the grave and slowly putting the other one in now.

I just hate putting money into my old computer.

I can't agree more.  However, this one might be worth an SSD upgrade. You can get a 120GB for under $20USD and a 250GB for under $40. Why go through the trouble?
Here's a little teaser....
This is a little side project I've been hacking away on... It's a white 2007 MacBook.

(Forgive the bezel, I'm dealing with an unscrupulous vendor in NJ who sent me a dead LCD and refused to exchange it due to COVID-19 and I'm waiting on a resolution from AMEX). I'm into this for about $40 (a little horse trading on ebay helps) and everything works - Ethernet, Bluetooth, external display, and even WiFi!  It's utterly useless, from a macOS perspective, but not for FreeBSD with the MATE DE. Even if all else fails, I still have a good SSD that I got for $20 that can go into another machine...
Your iMac is a tank, and I'd hate to see it meet an untimely demise.  ;-)