The reason these measurements take long (total time per resistor) is temperature settling and multiple measurements per resistor to build average with the MI bridges, this way you can achieve this accuracy. If all this makes sense for that unit is truely questionable, given its instability. It would have been much more valuable, having acccess to such accuracy that cheap, to use a set of precision hermetic resistors with low drift. Not sure what the 5450 has cost, but probably in the same range as a set of such resistors from VPG.
I have my 732 calibrated to 0.3ppm, and they do it over a couple of hours in a tightly controlled ambient, somehow justifying the price.