Thanks, again, Jim. Audio analysis is off, I don't know how to prevent thumbnail searches while importing, but they don't seem to be the issue. What's odd is that import flies through the first 50 files or so and then slows to a crawl as slow as 1 file per second, then it flies again for a few files, and so on. Seemingly, one of the file types that I import is causing the issue. I have tried limiting import to only the types of audio that I own; FLAC, DSD/DSF, ALAC, WAV, AC3. I have tried importing a smaller set of files.
It simply seems to be something that changed in the import function after MC 20. This issue affects my 2017 iMac with a clean install of High Sierra, my older iMac running Sierra, and my older MacBook Pro running El Capitan. All have plenty of RAM, fast SSD boot drives. These we all tested trying to import from a folder structure on a fast, Thunderbolt connected external physical drive. Invariably, MC 20 imports the roughly 13,000 files in under 8 minutes. MC22 and MC23 (I don't have a license for 21) take hours. I've attached a screenshot of the import summary from MC20, 12,885 files in 7min. 24sec. I've also attached a summary for MC23, which I stopped after 24min. 23sec. at 1,907 files.
I'd much prefer using MC 23, since it supports retina displays, and offers more features. Fortunately, I don't import lots of new files regularly, but the developers at JRiver should be aware of this issue and, hopefully, remedy it. Is there somewhere on the site where I can report this issue?