I'm getting strange results when testing. If I drop a prefab with a moderate amount of text into the scene, I get "normal" lag until I edit the text, at which point it starts locking up. Then if I unpack the prefab, it's back to normal. The more text I add to the prefab (beyond the amount initially there), the slower it gets. I recorded a video demonstrating this - do you have a support email I can send it to?
The one consistent thing is: whenever the text in the STM text box turns bold, it starts locking up unity. When I edit the text as described above, it turns bold and this happens. For my tutorial prefab, I notice it's already bold when I try to edit it, meaning everything is super slow right from the start (even just opening it in prefab mode).
As for the auto pagination script: I couldn't get it to work. If I set it to auto it does nothing. If I click manual setup, it creates one empty STM object. I couldn't get it to overflow text into other objects no matter how much text I added.