So here is the simple-minded solution (as we discussed by phone).

I used Total Commander to change the modified date of the Include file. I moved it back a day. After that the Build settled down to 2.5 seconds. I didn't test backdating by an hour or a minute.

Still a mystery to me why this works, or alternatively, why the Build does not get faster automatically a day later when the Include file is a day old. Yet what I have been seeing is that once the Build gets stuck on slow it would stay that way for days or weeks.

Fun and games.

Peter