iMac on High Sierra has ground to a halt after loading Adobe Lightroom

Hi

I have a Mid 2011 iMac running 10.13.6 High Sierra, which we just upgraded to 16gb of RAM. It was running extremely well, able to work simultaneously in three large Adobe programs...UNTIL I downloaded and installed Lightroom CC. I started consolidating a LOT of photos, and it quickly filled up the internal 500 gb HD, so I moved the catalogue over to a 2TB external drive and deleted it on the internal boot drive. But the machine seems to have gone into shock; Finder is very slow or unable to execute commands, and large programs are not loading at all.

I ran First Aid, which reported no problems. I ran Etrecheck, which shows "below average." Copy attached. I really do not know how to read these -- and humbly submit it here in the hopes that someone out there can take a look and let me know if there's a problem I can fix. It's on hourly Time Machine back-ups, so I can if necessary wipe the drive, maybe upgrade to Mojave and thin out some of the applications. I tried to list those here, but the list won't copy -- it is huge. There was a weird quirk when I went to load the list of apps: the system profiler took forever, and there are TWO of every app. There are two users who use this machine, with separate accounts -- maybe I somehow duplicated all the apps? Or

Thanks to anyone who can lend a hand. This has gummed up some very creative work that I really need to get back to!

Rob

Hardware Overview:

  Model Name: iMac

  Model Identifier: iMac12,1

  Processor Name: Intel Core i5

  Processor Speed: 2.5 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 6 MB

  Memory: 16 GB

  Boot ROM Version: 85.0.0.0.0

  SMC Version (system): 1.71f22