Unity's own inspector doesn't even support RTL text, ligatures, or Emoji, (I've already made packages for fixing these in-editor, but the inspector's text areas seem untouchable... Look up any "how to type RTL text in Unity Editor" question and every answer involves typing LTR and fixing it in post) so I don't think any optimization has been done on it in the last 10 years either... I'm thinking of ways to optimize this right now and everything just hits a roadblock besides hiding the text field itself. The unity editor just introduces a lot of overhead typically.
I'll write the multiline auto-pagination script asap (that would be a really good sample script for a common use-case, I want to make it) and see what can be done in terms of optimization for large fields of text on their own, but there's only so much that can be done with hundreds of Vector3s being updated on a CPU-bound mesh every frame. Splitting it into separate coroutines like the multi-mesh solution could be a good hint towards that though, if I get a free week sometime I would love to mess around with that. (e.g. instead of one coroutine for reading a mesh/maintaining animations, maybe a coroutine for each character could perform better...? Will need to test)
I originally developed STM on a 2009 macbook pro, then a 2017 mid-range windows laptop, so I'll admit I'm a bit curious about your system specs! I really do enjoy optimization, but it was consuming my life years ago, so my approach is now "if this ran fine on a 2017 iPhone, it'll be fine on anything now", haha. That said, it's been a while since I've done a peer code review, so I may pay a friend to look over my code to see if I missed anything. A couple features were added in the last few years, so maybe something got left out.
I'll take no offence to a refund, all that matters is getting your game done in a timely matter... I spent the better part of this last decade developing STM to save other developers' time, but I just can't pour quite as much into it anymore between contract work and personal life... but I will see what I can do over the weekend! STM is my baby, I want to make sure it 's still working as it should.