Complex sentence: The achievement was traditionally regarded as the hallmark of a world class sprinter, but its significance has become less important since the late 1990s as an increasing number of runners have surpassed the ten seconds mark.
Split into simpler sentences: The achievement was traditionally regarded as the hallmark of a world-class sprinter. Its significance has become less important since the late 1990s, as an increasing number of runners have surpassed the ten seconds mark.