On the other hand it could be distracting to get something happening in the screen while you are thinking the best way to form your query. Usually when I am about to ask someone a question and that person tries to give me the answer much before I have finished, it can get a bit annoying.
So I am not convinced this change in speed will bring much benefit, and it risk (from the sound of it) to be distracting. However this is just my gut feeling. I'll still give it a try, and I am sure other people will have different opinions.
It's good to see Google trying to improve the search interface in any case.