It only seems to slow it down once before it cuts the audio, but that seems to be half speed. You can also speed it up to play it faster. It does seem to preserve pitch, so it is useful for trying to figure out a riff. Not as fine grain as real audio software, but a quick trick. (As well as some other useful YouTube tricks).
I also make playlists in certain keys so I can play a bunch in a row on one harp.