The preview uses your browser's pitch-corrected playback, so you can test a speed immediately. Export loads a WebAssembly build of FFmpeg only when you create the result, then applies time stretching and writes a lossless 44.1 kHz stereo WAV.
Changing tempo is not the same as changing pitch. A 1.5x setting shortens the duration to two thirds of the original while keeping speech and music near the same perceived key. Browser memory makes this best for short or moderate files; very large or damaged audio may need a desktop editor.