How should we test the hypothesis?

You are interested in the presence of lane bias toward higher lanes, presumably due to a slight current in the pool. A natural null hypothesis to test, then, is that the mean fractional improvement going from low to high lane numbers is zero. Which of the following is a good way to simulate this null hypothesis?

As a reminder, the arrays swimtime_low_lanes and swimtime_high_lanes contain the swim times for lanes 1-3 and 6-8, respectively, and we define the fractional improvement as f = (swimtime_low_lanes - swimtime_high_lanes) / swimtime_low_lanes.