J1 simply because it accelerates the production of ground units on the mainland.
J1 - Build Minor and place in Kiangsu
J2 - Build 2 Minors, place in FIC and Kwangtung. Build/Place 3 MEC in Kiangsu.
J3 - Build/Place 9 MEC in Kiangsu, FIC and Kwangtung.
J4 - You’ve now got 12 MEC on the Mainland and another 9 to be placed (maybe some ARM too!).
J5 - You’ve got 21 MEC on the mainland, which is 4x the amount of units Calcutta has been puts out per turn out since J3.
If you wait until J2:
J1 - Build Minor and place on Kiangsu
J2 - Build/Place 3 MEC on Kiangsu
J3 - Build/Place 2 Minor on FIC and Kwangtung. Build/Place 3 MEC in Kiangsu.
J4 - Build/Place 9 MEC on Kiangsu, FIC and Kwangtung.
J5 - You’ve now got 15 MEC on the mainland which is 3x the amount of units Calcutta has been puts out per turn out since J4.
The difference is subtle, but the difference in units and the volume of them for two turns for Calcutta is approximately 10 ground units, of which you now have 6 less ground units yourself by waiting a turn. You also setup a much larger nut to crack on Yunnan meaning you have to fight potentially two large battles and win both convincingly to ensure Calcutta falls quickly.
That 1 round delay gives you a much smaller ground unit advantage in Asia which translates to more lost planes to take Calcutta (excluding the SB blitz people do from FIC now).
This is why I always subscribe to a J1 DOW and voted as such.