It took the S&P 500 712 days to double off its March 6, 2009, low. That, as noted earlier, is the fastest double on record.

    The previous fastest double, according to a brief survey by Capital IQ, is 890 days. That episode started July 30, 1996, when the S&P closed at 635, and ended Jan. 6, 1999, with a close at 1272.

    No. 3 on the list is 903 days, though this one comes with an asterisk because it covers a period that overlaps with the second-shortest double. It started Jan. 25, 1995, and ending July 16, 1997, with the index at 936.

    In any case, the new record is a 20% improvement over the previous one – which is either impressive or utterly nervewracking, depending on how you see things.