The defer delays the function call, but it does not delay evaluation of the call’s arguments. This means that both timestamps are therefore captured near the beginning of the request, even though the second line is printed when the handler returns. Also, time.Since can use the monotonic component stored in time.Time, so the duration is not distorted by wall-clock corrections.
Durability is not the problem, but because the state lives somewhere you structurally can't relocate. That constraint never shows up in a benchmark, it shows up in an enterprise procurement call.