Yes, this is a possibility, with some caveats.
If I want to use Micropub to upload images (or other media) for inclusion in a post, those images should be made immediately available to the Micropub client. That is to say, the media upload process should put them into the live site, not the source of my site. This is actually relatively easy to accomplish; and indeed I’d likely put the uploaded media into both the live site and the source’s static directory.
I’ve revised my theme a bit, making use of this forum post to disable RSS for sections. This is what @kaushalmodi was suggesting above, but this forum post cleared up much of my confusion.
Interestingly, even though Hugo is now generating several thousand fewer items, it takes about the same amount of time: ~4 seconds on my laptop, and ~13 seconds on my server.
I’m happy enough with these changes, even though they don’t really improve the speed. 13 seconds isn’t terrible.