Fast Dormancy is a network feature which helps limiting the amount of signaling between the phone and the cell network. It’s meant to improve smartphone networking and battery performance. But, when Fast Dormancy it is not enabled in the network and is enabled on the phone it ironically works the other way around, and actually drains more battery than before.

You can disable it by following these simple steps:

1. Dial *#9900#
2. Press “Disable Fast Dormancy“
3. Press “Exit“.

Source: XDA Forum