iMac is running extremely slow after clean install

I have a 21.5" late 2015 iMac that I've been having trouble with. It's very slow, and beachballing on a regular basis, startup is extremely long - you get the idea. So I backed everything up, formatted the drive and did a clean install. But after the clean install and before installing most of the applications I had on there before, I noticed that the startup still takes forever and that the whole thing is running really slow again. Tried First Aid, Diagnostics - nothing to show.

I tried running Etrecheck and the results were inconclusive. I've attached the report below.

I also found a diagnostic script written about in this post and ran that. Those results are below

If anyone has any ideas on how I could speed this thing up, I'd appreciate it. Thanks!