The Use Dither option turns on a preset, noise-shaped dither function that improves audio performance when fading in or fading out of silence, and crossfading between low amplitude clips. Dither is usually not necessary when applying fades to clips with audio at high amplitudes. You can disable Dither while editing fades in the Fades dialog to speed up previews and fade recalculation, then re-enable Dither to create the final fade or crossfade.