Google Checkout will impose a 15 min. delay on any order which contains tangible goods (products which have Shipping enabled), due to their policy of allowing buyers to cancel such orders within 15 min. after checkout.
If you are not shipping any tangible goods, you can bypass this delay by disabling Shipping in all of your products' settings. Orders which contain only digital products (items without Shipping enabled) will not be delayed.
For more details, please see Google Checkout's cancellation help page here