What actually makes a nightcore edit
The speed control resamples the audio rather than stretching it in time. Playing a file faster than it was recorded compresses every waveform and raises its pitch along with the tempo, the same way a vinyl record sounds when you nudge the platter speed up; that pitch rise is the whole point of nightcore, not a flaw to correct. Resampling is exact, so the speed-up itself adds no artifacts.
The brightness control is a treble shelf filter, boosting frequencies above roughly 3 kHz by the amount you set, up to 6 dB. Since speeding up a track already pushes its frequency content upward, the extra treble on top is what gives nightcore its signature thin, sparkly, almost sugary top end, especially on vocals and hi-hats. Both steps are cheap to run, so even a long track renders in a few seconds, and Play preview lets you hear every slider change live.