>which means that optimizing a function to make it faster is a breaking change (what if suddenly one of your queues clears too fast and triggers a deadlock??), despite the fact that 99.99999999% of your users would probably appreciate having code that runs faster for no effort on their part.

I agree with your point, but that's poor example because you can't rely on function's speed reliably and easily.

Timing differs between hw, OS, OS updates, whatever.

Meanwhile it is trivial and easy to rely on error messages.