There is a lag that is often, but not always, present when using Nuance voices. It is most evident if you type some text, then delete character by character. Instead of responding immediately you will often hear the tone (if you have VoiceOver set to play a tone), then a pause, then the letter. It can also be seen when switching apps, or trying to do several commands in rapid succession. Note that some users report no such lag with Nuance voices, so your milage may vary.