Ok--it seems fixed now. I'm posting this in case others have the same issues. Here's what worked on my Windows 10 system:
--I set up all the Windows Defender exceptions. Then I set up Trend Micro to leave JRiver alone, too (set as an exception for the same locations as Windows Defender). In my case, this did make the program *initial* startup much quicker, which is nice.
But the AntiVirus stuff did not fix the UI problem: JRiver would still hang for 30s + on sequential and simple tag operations (like updating a title). --I noticed that when responsiveness returned to JR Media Center, my Windows desktop would "refresh" all the icons. This, to me, was an indicator that Windows Explorer was getting really messed up and restarting. Would have been an BSOD years ago
But in any case, it seemed like a UI rendering issue to me.
--After digging through the forums, I found a clue about using Chromium or IE engines as web browsers in JRiver. So I switched from Chromium, to IE, and restarted the program. The problem was still there. However, when I switched back to Chromium (and restarted JRiver), the problem vanished.
So perhaps there was an upgrade issue from MC 26 to 27, probably caused by AV identifying the uninstall exe as a virus, that prevented everything being set up properly in MC27.
Thanks for the advice on AV, by the way. I'll consider it, but with kids around, TrendMicro provides more than just straightforward AV protection, and it seems to work well again