Lyxen
Great Old One
I'm not, though. I can perceive the beginning of a spell being cast, just like you claim the disappearance is the beginning it teleporting.
And if you cast dispel magic when a spell starts to be cast, you will have nothing to dispel either, since the spell is not yet present, so it's not a problem.
That means that my trigger for the dispel magic happens before the spell us cast.![]()
And therefore there's nothing to dispel.

Counterspell isn't any faster than a readied dispel magic. Both are reactions.
Haven't you caught up on yet on the fact that some specific reactions interrupt their triggers while some (those of readied action) occur after the trigger has completed... sigh
With a description that starts with "You attempt to interrupt a creature in the process of casting a spell." Guess which one counterspell is ?