swifter apply gets stuck in timing phase

Question

I am trying to use swifter.apply to speed up a standard pandas lambda function, which is otherwise working fine, but addition of swifter causes the function to stall. Delving into the code a bit, it seems to get stuck at this line:

timed = timeit.timeit(wrapped, number=N_REPEATS)

which I assume is timing 3 standard iterations of the function to determine whether parallelisation is quicker than a standard application.

This is the code I use to apply the function:

df['start_time'] = pd.to_datetime(df['file'].swifter.apply(lambda x: start_time(x)))

And this is the function (reading part of a text file):

def start_time(file_in):

    fp = open(file_in)
    for i, line in enumerate(fp):
        if i == 2:
            break
    fp.close()

    return line.replace('\n', '').split(',')[1]