Last time, push notification was the culprit.
The new theory is that it’s a bug in the location services/system services. The automatic time zone finder apparently runs more or less constantly, pulling 3g signals and draining the battery. I turned off that function.
fast-battery-drain
that happens when there are UFO’s around you ,eyes on the skies
This time too.
And I think dramatic is just that. Most users report no difference. Some notice a little. I haven't noticed any.