Performance Troubleshooting? Hardware issue?

We inherited a 2013 imac 21.5", that I've upgraded with 16GB RAM and a 2TB Crucial SSD. It should work great, right?

Performance on everything is absolutely terrible on every OS I've tried. We get a lot of bouncing balls and things are SLOW. Copying a 1GB file can take several minutes - it should take seconds. It can take 30s to display a directory in Finder; it can take 60s to switch applications or windows.

I didn't activate trim, but I do have "put disks to sleep" as "off" and left it on the boot select screen overnight to ensure the drive's internal defragmentation works.

I've booted into the hardware test mode, and everything passes. I tried running rember (memtest), and ram is fine. Everything I've tried on the drive says it's fine. I can't figure out WTF is going on.

Does anyone have tips on what I should be looking for? I've used macs for 30+ years, and this makes no sense to me. I'd at least expect to see the hardware test show a drive connection or bus issue.

This is not OCLP's fault. I did try installing normal Catalina, and had the same issues. I'm just hoping someone here might have pointers.