No rhyme or reason I think
Without any advance planning I wound up making two transfers this evening in short succession and am happy to say that both went through instantly, one for approx 24,000 pts and then one for 13,000. (Had I planned it out better I would have combined both into one transfer). I'm certainly not complaining, but after following this thread (and a similar one for Aeroplan) I'm starting to think that it's pretty difficult to find consistent patterns in what transfers instantly and what doesn't. It seems more like getting randomly selected for extra TSA screening. May the next person be as lucky!