In the Metadata panel in the right column of Library, is the dropdown set to Default?
A couple of years ago, LR could slow down considerably when that was not set to Default, though that was fixed for most people a long time ago. Perhaps your problem is related.
Another troubleshooting step: Try setting Metadata > Show Metadata For Target Photo Only. Does that alleviate the symptoms?
Using Default helps some, but the setting for Metadata>Show Metadata For Target Photo Only is what solved the problem. Thank you!
the setting for Metadata>Show Metadata For Target Photo Only is what solved the problem.
There's clearly something bad happening with your LR, since it shouldn't be necessary to select this setting. Note that if you ever want to change a metadata field for a batch of photos at once (e.g. caption or keywords), you'll need to uncheck the setting.
That's not good at all! The main reason for selecting multiple images is to update keywords.
So what in the world is 'something bad'? Any ideas? Would you recommend uninstalling and reinstalling? I don't know that I've done that. The update from Win 7 to Win 10 worked quite well, and this problem predated that upgrade. I did not have to reinstall any of the Adobe programs, that I recall.
The metadata is what I watch to see that LR is 'done' since the display changes to show <mixed> in the appropriate fields. But isn't the metadata stored with the images on the disk? Or is it in the catalog? If it's either of those places, then the program should simply be reading it. I have optimized the catalogs (i have several) to see if that would help, but it made no difference.
I don't know any reasonable guesses as to what might be causing this. You could try uninstalling / reinstalling, though that doesn't fix many problems with LR and I'd guess it wouldn't help here. You could try making a new test catalog, import all your photos into it, and see if the problem still happens with the fresh test catalog. (But beware that it can take LR hours to rebuild previews if you have lots of photos.)
If you keep Metadata > Show Metadata For Target Photo Only selected, you could use Metadata > Copy Metadata and Paste Metadata to copy keywords from one photo to selected photos, or you could use the Painter tool. Neither is quite as convenient, though.