The first introduces a character who is flashing back to their worst experience ever which was the time he/she realized they were dead. If this is the inciting moment of the book, you’ve nailed it in this sentence. The only thing I would do would be to re-word so you don’t say “moment” twice.

The second one implies no feeling on behalf of the character (generally not as strong since editors always want to know the effect/feeling of actions on the characters) and it may imply that the main character is a little daft (since you are saying it took them a while to figure it out).

And that’s my two cents (or one cent if you take into the exchange rate!)

But also without knowing context, it is hard to say; if she was a British Professor of Literature then I can see her saying to herself, ” . . . anything but instantaneous.” If she was typical American young woman, the first sentence is more believable.