What actually makes a slowed + reverb edit
By default the speed control resamples the audio instead of stretching it in time. Playing 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 the sound people recognize as slowed. If you want the slower tempo without the lower key, switch on Keep the Pitch and the slowdown runs through Signalsmith Stretch, a phase-vocoder time-stretch engine, instead.
Reverb is a set of short, decaying echoes (60, 120 and 240 ms) mixed under the dry sound, which suggests a room 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. A limiter at the end keeps the echoes from clipping.