Personally, I'd prefer to keep the code simplicity, and the speed for small inputs here. Python's needs aren't the same as SymPy's needs or SAGE's needs, and not all of the fractions.Fraction use-cases involve summing lots of values with incompatible denominators.

> With big denominators (~10**6) this really does matter, my local
> benchmarks suggest the order of magnitude difference for
> summation of several such numbers.

Could you give some idea of the crossover point for a single addition? At roughly what size numerator/denominator do we start seeing a performance benefit?