I have the same problem from time to time. Now I found this:

Go to Apple menu > System Settings from the menu bar and select Desktop & Dock in the sidebar. Then, uncheck Displays have separate Spaces under Mission Control. For this change to apply, log out and back in.

Let’s see if it helps.

I don’t know if that makes sense, but have you tried to put the outlined text on a separat layer?

One of the last books I have worked with was about 600 pages with hundreds of photos and I didn’t have the feeling that InDesign would slow down or have any problems. But I had a lot of chapters, each in its own text chain and none of them longer than maybe 50 pages or so.