My experience:
As long as you open Google Wallet and put your PIN code in before you order at the register AND, as long as you don't try to jump the gun and use your phone before the cashier is finished ringing you up, it's WAY faster than using a credit card.
I did it at a BK yesterday. Ordered, got the total, turned the screen on, tapped the phone to the credit card machine, transaction went through in less than 3 seconds. Simply amazing, IMO.
Did it at Rite-Aid later. I hadn't unlocked Google Wallet, so when I tapped, it opened Google Wallet and asked me for my PIN. I had to type it in, then tap it again to finish. Still took like 15 seconds. Not quite as amazing, but was still better than the woman in front of me. And I learned my lesson. Open Google Wallet right before it's your turn.