'Why?' is a very powerful question.

A few months ago I heard a talk by the chief product manager at a photo album printing company where he talked about an in-depth series of customer interviews they'd recently conducted.

His advice was to keep asking why until you get to the root of why someone did something. However, if the answer you reach 'Because I don't want to die alone', then you've gone too far...

If you use a stopwatch or equivalent, the user may get weirded out. Maybe do a rough-count in your mind and take a note, but to emphasize time above other factors may be a mistake, except for special domains or circumstances.