Hmm does the text mesh have any animating text on it? If so, Super Text Mesh calls SetMesh() every frame... Usually a game will only have one large text box at a time that will have animated text, so this is normal.
But if you're just talking about in-editor... I have some code that forces Super Text Mesh to call Rebuild() (a heavier call than SetMesh() ) whenever the size of its rectTransform changes. This is also invoked upon OnValidate(), and OnEnable() which could be why it's being called when parent rectTransforms change. Both the rectTransform size and OnValidate behaviours are for ease-of-use in-editor, and don't actually run in a built game. These two are called under STM's Update() cycle and could potentially be commented out?)
If the text mesh does have animated elements, this might be effecting performance, too...
I could potentially add a toggle in the future to stop STM from auto-rebuilding in-editor, because I can see how it would be kind of annoying for larger text boxes...
I hope this information helps! A feature like this could be useful to have... if I'm understanding the problem correctly, haha.