I think in the original webnovel she actually did a hyperspeed neck crack in front of him which is why he was so weirded out. A shame the artist changed that.
@Emerald_mage It kinda just looks like she was posturing that she could do it, with her hand on her chin like that. I was expecting her head to become a blur or something. I guess it was supposed to be so fast that the reader can't see it.
@Azebu Epsilon is a famous pianist with "original" songs, having received awards and invitations. There ought to be thousands of people who know Moonlight Sonata in that world already.