I realize that I can have it updated on every change made, but that really makes for an appreciable slowdown in operation. I'd really like to be able to either tell Lightroom to update all the files I've fiddled or else readily select all images with out-of-date XMP files and press CMD-S. I've not been able to find a mechanism to do that. Am I missing it, or is Adobe missing it?
That option is not available as far as I am aware, Lightroom can be set automatically write to XMP as an option. When you are working in Lightroom as far as Lightroom is concerned the info is in the Catalog file. Lightroom does not Automatically read from the file. If you are aware of the folders you are working with select all the images and CMD-S. It does not matter if the files you select do not have any changes.
The answer I was afraid of. 😞 I'll just have to remember that if I'm going into a jump-around-the-catalog session, I need to turn on automatic instantaneous XMP saving. If they could just change it to periodic it wouldn't be such a system drag. Thank you.
I am 99.999% sure that writing to xmp does not cause noticeable slowdowns. It is a trivial amount of data that is written. There's no way that writing such a small amount of data will cause Lightroom any speed problems.
So in order to help further, we would need to know: Exactly what actions are slow?