John,
the slow X-sync is a result of the hyped silent, horizontal moving shutter. When the camera actuates flash, the whole 24x36mm frame has to be "naked". This means that the 1st curtain must have travelled the whole 36mm, but the 2nd curtain must be still in starting position. At anything shorter than 1/50, the 2nd curtain starts before the 1st one reaches the end.
Nevertheless, if the to-be-illuminated-main-subject is in the middle of the frame, but not at the edges, you can flash at shorter exposures. Try out if 1/125 works well, I think anything shorter will not.
Till