Reinstalled 2.87.03, works fine. Redownloaded 2.90.02, install file is identical to previous download of it. Reinstalled, problem is the same. From status line: takes 5 minutes to scan all directories. Biggest dirs (with 1-2 k file count) take one minute each to scan. But it's not the download time that this the biggest difference. v2.87 takes over 2 minutes to scan all dirs, because I haven't eliminated caches and such from the FARR scan path. The biggest diff is that when 2.87 is scanning, it's highly responsive. When 2.90 is running, it is (totally?) unresponsive until it has finished scanning. Then when I press another key, it's back to square 1.
Only thing that's unusual on my system right now is that Diskeeper defragger is working in background, but (1) it doesn't bother 2.87, (2) it doesn't bother anybody else (it's running in deep background, very polite), and (3) I think that DK only works on a few files at a time, and 2.90 is sluggish on all directories, every time. Oh, and it's also very slow (tens to hundreds of seconds) to respond to a right-click on its tray icon, vs instantaneous for 2.87
Reinstalled 2.87.03. I'll stick with it for now, may be able to re-test a new version but not much more than that until next weekend. Thanks mouser, and sorry to bear bad news.