Yes, it’s an absurd example to make a point. We don’t normally consider performance in scope for what’s considered a breaking API change and there are good reasons for that, including being non-portable. Performance guarantees are what hard real-time systems do and they’re hardware-specific. (There is also the “gas fee” system that Ethereum has, to make a performance limit consistent across architectures.)

But there are still informal limits. If the performance impact is bad enough, (say, 5x slower, or changing a linear algorithm to quadratic), it’s probably going to be reverted anyway. We just don‘t have any practical way of formalizing rough performance guarantees at an API boundary.