It does. MP3は普遍的な損失コーデックであり、AACやOpusよりもビット当たりの品質が劣る代償で、何もそれを再生しない. That determines whether the retimed result can be written back cheaply or has to be fully re-encoded.
透明度の決定は，透明度の変更を行う段階である。 That is the step most people actually came for, and doing it after MP3 の速度を変更 rather than before it means the conversion works from the file you finally settled on.
