Occam's Razor. The answer with the fewest assumptions tends to be the right one. If we go Doton we have to assume Sarada has learned Earth Release off-screen (despite never once even hinting at its use anywhere else before or after this scene) and that she mastered it enough to counter a specialized Raiton user's lightning, plus her having extremely hand seal speed to counter it. If we go speed, its just her being extremely fast.
Therefore I say speed.
I agree that sounds more crazy, but the actual animation makes it completely look like the lightning hit a wall that was not previously there.
I agree it is crazy to think she would have earth style, and could use it like that, but animation-wise, nothing hints she dodged and hid behind a wall, and something DOES hint that the lightning hit a wall that was not previously visible.
Maybe genjutsu? probably not, but the alternatives are just as crazy honestly. I can't really see her dodging that electricity at point blank range so fast we don't even see her, and i can't see her activating a earth jutsu, especially at that speed, but one of these things HAD to have happened.
