According to Google:
At Google we are obsessed with speed. Our mantra is faster = better. This is true for our Google Voice mobile apps as well. When you want to make a call, your phone should connect you as quickly as possible, whether you're calling via Google Voice or not.
Today we are launching an enhancement to the Google Voice mobile app on Android and Blackberry phones, which makes placing calls much faster. We call this feature "direct access numbers."
Read how they made it faster here