Time-shifting requires very intensive realtime calculations, and the JS-8's brain has to crunch thousands of numbers per second on-the-fly when the tempo is changed. Depending on the complexity of a phrase's audio, the sound quality of the time-shifted result will vary, with the best results being achieved with less complex material.
Another factor is how far the audio is time-shifted from its original tempo, with better quality the less it's shifted. Keep these considerations in mind to achieve the best results when time shifting.
Another factor is how far the audio is time-shifted from its original tempo, with better quality the less it's shifted. Keep these considerations in mind to achieve the best results when time shifting.