I suspect the performance part is only true if you're familiar with Python's stdlib performance quirks, like how `datetime.fromisoformat()` is orders of magnitude faster than `datetime.strptime()` (which would likely be the function you'd reach for if not familiar), or at the very least, that directly casting slices of the string into ints is in between the two. This example is parsing a file of 10,000,000 ISO8601 datetimes, then counting those between `HH:00:SS – HH:02:SS` inclusive. The count method is the same, and likely benefitting some OS caching, but the parse times remained constant even with repeated runs.

    $ python3 times_python.py strptime_listcomp
    parse time: 45.96 seconds
    count time: 0.54 seconds
    count: 498621

    $ python3 times_python.py slices
    parse time: 9.96 seconds
    count time: 0.40 seconds
    count: 498621

    $ python3 times_python.py isofmt
    parse time: 0.80 seconds
    count time: 0.38 seconds
    count: 498621