Google Checkout response and "Processing orders automatically"

Question

I have selected "Processing orders automatically" on Google Checkout and it credited the amount automatically. But it takes few minutes (sometimes upto 15 minutes) to get response on the Call Back URL (notification url) from Google regarding the success state.

So the user need to wait upto this time to get autorization and is very bad. How can speed up the process ?

Currently I check new-financial-order-state=CHARGEABLE and authorize the user. How can I speed up the process? Is there any other response values to allow user without waiting much time?