If the thing you're timing is expected to have a constant running time, the only thing that can slow it down is external factors (e.g. OS background tasks). Be

If the thing you're timing is expected to have a constant running time, the only thing that can slow it down is external factors (e.g. OS background tasks). Best-case over a large number of runs is the correct way to approach the ideal running time of the task in this case, as you can eventually hit a run that didn't get impeded by anything.