I worked out a slightly speedier version of decimal_to_ratio today (Stefan: that's when I duplicated your bug report):

from decimal import Context

def _decimal_to_ratio (x):
    _, digits, exp = x.as_tuple()
    if exp in _ExactRatio.decimal_infinite:  # INF, NAN, sNAN
        assert not d.is_finite()
        raise ValueError
    if exp < 0:
        exp = -exp
        return int(x.scaleb(exp, Context(prec=len(digits)))), 10**exp
    return int(x), 1

makes the variance functions in my re-implementation about 20-30% faster for Decimal. It's not a big breakthrough, but still.