Before, because you follow the order of events the spell describes... Maybe you should read the Spellcasting section of the PHB to get better acquainted with the subject you want to discuss?
You know, after the thing with the "Make an Attack" rule a while back, I'd think you'd be more careful about snide comments like that.
This is how you resolve the spell:
1. Declare you're casting Chain Lightning
2. Spend the action type required (most likely your Action for the turn)
3. Spend any material costs and/or other requirements in order to cast the spell (V, S)
4. Complete casting the spell. (last chance to use counterspell)
5. Resolve the effect(s) of the spell (the spell is now cast and it's effects are being resolved)
6. Declare legal primary target within range of the spell's effect
7. Declare legal secondary targets (up to 3 for the base spell, more with higher spell slots) within range of the spell's primary target
8. Have all declared targets roll a saving throw
9. Roll damage
10. Apply damage to all targets who failed their saving throw OR half that damage to all targets who succeeded.
Unless a spell, item or weapon attack specifically states that you resolve the attacks simultaneously you never do because you follow the Making an Attack rules. The only exception in the game is Magic Missile AFAIK which actually does specifically call out that you resolve attacks simultaneously. Which again, is exactly what Jeremy Crawford said in his earlier tweet:
That's an interesting thing. See, you said here:
6. Declare legal primary target within range of the spell's effect
7. Declare legal secondary targets (up to 3 for the base spell, more with higher spell slots) within range of the spell's primary target
8. Have all declared targets roll a saving throw
Why isn't it:
6. Declare legal primary target.
7. Have primary target roll save. Roll damage, apply damage.
8. Declare legal secondary targets.
9. Have secondary targets roll save.
Or even:
8. Declare first secondary target.
9. Secondary target rolls save.
10. Declare second secondary target.
11. Secondary target rolls save.
Personally, I would go for the first because it's streamlined, but there might well be cases in which your preferred choice of secondary targets is contingent on knowing the outcome of the damage to the first target. And it seems obvious that intent is that you can't do that. But that means we have some instantaneous spells (like eldritch blast) where we're assuming that you can make each attack sequentially, after seeing how the previous attack or attacks worked, and other instantaneous spells (like chain lightning) where you declare all targets at once.
One rule might be "instantaneous implies simultaneous", meaning that every instantaneous spell requires you to pick targets up front, then resolve effects on them, and the effects are considered simultaneous. Another might be "attack rolls are always sequential, otherwise spells are simultaneous unless stated otherwise", so you pick multiple targets up front unless the spell has attack rolls. There aren't many spells with multiple targets and attack rolls, and I've seen different rulings on some of these topics.
(See also the question, in prior editions, of whether targets that make their save against a spell that affects N hit points or creatures count against its total target count; after all, they are "not affected" in some cases.)