>Helper scripts using ls can involve lots of items and recursion, so the slowness can go exponential.

It can't go exponential. Recursion or not, you did N passes with ls, and you'll do N passes with exa.

Unless exa is exponentially (not by a linear factor) slower than ls, the slowness can't go exponential.

Scripts using ls should keep using ls whatever you use for interactive use. Though more often than not such scripts ought to use something else like find to begin with.

Even the most strident fans of exa wouldn't advocate replacing ls with exa. We just set it as an alias which won't affect script execution.