The real value of someone being able to respond on the time-complexity of a given arbitrary function is that it shows they can build a good mental model of what

The real value of someone being able to respond on the time-complexity of a given arbitrary function is that it shows they can build a good mental model of what goes one when the function is run. Building a good mental model of a program is the most important thing when designing it, building int, extending it, or fixing it. So it's not about performance - it's about understanding and abstraction skills. All good things to test for in an interview.