What actually makes a slowed + reverb edit
The speed control uses FFmpeg's asetrate filter, which changes the rate the audio is read at instead of stretching it in time. Reading a file slower than it was recorded drags every waveform out and drops its pitch at the same time, exactly the way a cassette sounds when the motor runs down; that pitch drop is not a side effect here, it is the point, since it is what people recognize as the slowed sound.
Reverb comes from FFmpeg's aecho filter, which simulates a room by feeding the sound through a network of short, decaying echoes rather than one obvious repeat. The Soften the Highs toggle applies a low-pass filter on top, cutting the sharpest treble so cymbals and sibilance blur into the reverb tail instead of cutting through it, which is what gives the genre its underwater, tape-warped feel.