I tried turning off auto save and it doesn't really make a difference (it happens both in prefab mode and in scene mode when working with prefabs). I made two more strange observations:
1. The slowdown seems to heavily depend on the text content. If I copy/paste "Hello World!" over and over it performs better and I can get about twice as many characters before it slows down to the same level as my tutorial text (which is ~900 characters). The tutorial text is just plain English text, nothing fancy.
2. With the repeated text prefab, If I save the prefab and then drop it into the scene I get the laggy, but still usable behavior; however, the moment I touch the actual text, it turns bold (in the inspector window) and starts locking up unity.
Note that when I say "usable", I mean I can edit the text settings. It's still pretty unusable for typing text into the text box.
EDIT: I'm also noticing that after recompiling Unity takes over a minute to import any prefabs that have a STM object (but only if they're in the scene). I also tried making the text mesh non-serialized and it made no difference.