My experience is it is basically immediately after you say the word, there's a split second of delay, so far I have had zero issues with it, although sometimes it won't hear me when I speak it very softly, when I speak loudly and clearly it always works great.
Now, if it's any more instantaneous than the button pressing, I think it's really about the same. What I find the most nice is that they are just all available without switching around or scrolling through your favorites. It is very nice and has been completely smooth for me.