Why complexity matters

Timing one run tells you almost nothing.

Scale, not speed

Code that handles a thousand rows in a blink can stall on a million. Complexity predicts that jump without running it, which is the whole point of the notation.

Hardware makes constants smaller. It does not change the shape of the growth.
The interview question is really: what happens when this succeeds and the data grows.

Measure too

Analysis tells you which approach can scale. A profiler tells you where the time actually goes today. Good engineering uses both, in that order.