Can you explain how screen reader support could possibly have a noticeable performance cost?
The screen reader code should be doing absolutely nothing if it's not enabled - and even if it is, I can't imagine how it could affect performance anyway. For plain text, such as a terminal, all it does is grab text and parse into words (and then the part where it reads the words, but that's separate from the terminal) - I don't see how this is any more difficult than just taking your terminal's array of cell structs, pulling out the characters into a dynamic array, and returning a pointer to that.
That being said, is anyone aware of a significant missing feature that would impact performance?
[1]: https://github.com/cmuratori/refterm#feature-support