Wow. I just figured it out. I want to share, not only because I am super excited about it, but also if it could help others.
What was happening: When I slowed the "crawl" down to 10% which is what it needed to look right, then the duration of that slide would automatically adjust. To 6 minutes long! And I didn't know how to adjust both factors to make it correct.
My revelation tonight: So duh, I looked at where the marker was once the text properly got to the top of the screen. And I thought, "Can't I just split that right there, and delete all the excess?" YES. It worked. It is so simple but I couldn't wrap my brain around it earlier.