And yet, we've been explicitly told that it does in fact have enough duration that the warlock can choose targets after seeing the effects of previous blasts.
So there we have it. It isn't that fast. It's been answered. "Instantaneous" does not mean what you think it means, according to Crawford.
So Crawford has decided the word instantaneous doesn't mean what it means in a dictionary.
Well, this is why older editions decided what they decided about ray and beam spells. They wanted to keep the meaning of instantaneous consistent. I wish Mike Mearls would overrule Crawford on this one. It's all kinds of stupid. What we do we know so far about this instantaneous eldritch blast and scorching ray spell. Let's look at all the ridiculous destruction of verisimilitude:
1. You have time in a chaotic battle to discern the effect of the beam on the target and then fire again at the same target if you don't get the desired effect. You can do this every beam or ray you fire.
2 Yet you don't have time to move between beams even a few steps.
3. The spell has an "Instantaneous" duration, so an opponent can't dispel your spell, even though he should be able to given you are taking time to aim and fire a beam, choose your target, and aim and fire a beam again.
4. You get to knock the target back 10 feet per beam, see where the target is each beam, and fire again for up to 4 beams.
So the spell isn't really instantaneous. That's why making you choose targets beforehand and making it simultaneous was always the wiser way to write those types of spells. Crawford apparently did not take that lesson to heart in this new edition.