What actually makes a nightcore edit
The speed control runs FFmpeg's asetrate filter, which changes the rate the audio is read at rather than stretching it in time. Reading 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.
The brightness control is a treble shelf filter, boosting frequencies above roughly 4 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 filters are cheap to run, so even a long track processes in a few seconds once it is loaded.