I have made a couple interesting discoveries when it comes to hatching:
1. Magikarp's egg cycles appear to be shorter than what Bulbapedia lists (without Flame Body I hatched eggs at 1159 steps, w/ flame body 579 and 671 steps, Bulbapedia lists a minimum 1530 steps). Bulbapedia lists egg cycles by approximately one too many cycles, I think.
2. Flame Body / Magma Armor Pokemon no longer need to be at the front of the party to speed up hatching.
*edit* some further research seems to indicate:
3. Egg cycles work more or less the same way as previous generation games. 255 steps decreases the egg counter by 1 to every egg in your party (256 seems to contradict my research).
4. Flame Body doubles the number of egg cycles that are incremented per 255 steps from 1 to 2.
5. The O-Powers seem to straight up decreases the number of steps needed to increment the egg counter. I'm guessing level 3 decreases it to 128, level 2 to 170, and level 1 to 204.
6. The step counter does not appear to reset when an egg is spawned, hatched, or when you acquire an egg, but further research is needed. This means when you acquire an egg it is possible to start with your step counter anywhere between 0 and 254. Which is why you can hatch multiple Pokemon at the same time.
Which means that with a level 3 O-Power and Flame Body, you increment 2 egg cycles every 128 steps; a long-hatch Pokemon like Chansey can be hatched in 2550 steps., whereas Magikarp can be hatched in as few as 384 steps, assuming you start with your step counter at zero.