Removing all formatting did help, a little, but it was still slow as molasses to respond. I then wrote a short VBA routine that changes 99 Enters to spaces and leaves only the one-hundredth one as a paragraph ending. Let it run all night, and the paragraph count went down from over 730,000 to 690,000. I'm not interested in waiting several weeks for it to finish.

I then used find/replace to manually change most of the Enters to spaces, using accompanying patterns, so I wouldn't change the entire doc to one long paragraph. That got the count down to 24,000 and pages down to 1400. A close and reopen to flush the toilet, and the new doc works normally, despite the number of character and words staying the same - just the paragraph and page count are more normal. As I was doing the find/replace, the response time got steadily better, and after close/reopen became completely normal. I didn't run any actual timing tests, so it's pretty subjective, but even subjectively, the difference between minute-long lags/hangs and instant response is clear enough.

Removing formatting helped some, but not enough. Disabling spell-check had no effect, which is understandable - word are spelled or misspelled the same way, regardless of how many paragraphs they are in.

I suppose I could reverse the edits and progressively go back to the quarter million paragraphs, running timing tests along the way, but that sounds like a lot of work for not much useful information. It's now clear to me that for a document structured this way, Word is simply not the proper tool for the job.