Generally, the format of your audio files should not influence the writing speed. What matters is your recorder speed and your media speed. For instance, some old rewritable discs can be written at max 4x so no matter if you use WAV or MP3, burning the disc will always take the same amount of time.
You should check your media's supported writing speeds as well as your recorder's. Also, if you haven't modified the sample application much, I believe it has a hardcoded speed of 4x in BurnIsoAudioCD. If you change this to 0, the disc will be written at maximum possible speed.