Over time I discovered that out the slowness wasn't limited to switching between outline mode and text mode. Sometimes Storyist 4 on the Mac simply freezes, often for more than a minute. If I wanted to keep working, I was reduced to a Force Quit of the application since it wasn't responding at all.
I limped along for a couple of months in the hope of a new release. Eventually, in the absence of any fix (or even a response to my original post), I settled on the simple solution of reverting to Storyist 3. Problem solved.