There are two speed controls and only one costs anything. The voice’s speaking rate is part of synthesis: changing it re-renders the audio and bills credits. The loop speed-up plays the finished recording faster and higher-pitched, the classic subliminal treatment. It is applied at mix time, so it is free to change as often as you like.
Two speeds: spoken rate vs. speed-up
Voice speed setting re-synthesizes the audio (costs credits). Loop speed-up speeds the finished recording and raises its pitch: the classic subliminal treatment, free to change anytime.