By default we have maxOutputLines == 1000. Once you get to 1000 elements it starts to splice the output to only keep the last 1000. But then the same array index maps to a different output entry component. Which means all of them get re-rendered at once.

By adding a persistent key to each shell output entry react won't lose track of or think that the components completely changed.

Just to note: The shell will still slow down gradually until it gets to maxOutputLines, but at least it won't break completely. The speed then stabilises at maxOutputLines because we cap them.