React part maybe. The rest is what any TUI that's using ncurses would do. :)

It really bothers me that most of the TUI harnesses are using 100% CPU quite a lot just printing stuff to terminal. Seems ridiculous.

I guess it comes from syntax highlighting/formatting, which is probably not done incrementally, but over the entire so far displayed block of output, recomputed from the beginning for each new streamed in character. Can't imagine anything else causing the rendering to gradually grind to halt when eg. thinking block is open in opnecode and updates get palpably slow as it grows.

Terminal output itself is fast and consumes almost nothing. You can have 60fps terminal apps that update content every frame and that consume almost no CPU time.