Previously on 4.3 I had a very decent battery life ~7 hrs on screen for about 3 days on battery.
Once Kit Kat was out for my region I downloaded whole firmware and flash it through PC odin and wiped cash and data/factory reset after flashing.
The phone became more responsive and smooth though I faced a huge wakelock and consequently massive drop in battery life.
I also got another problem that once I wake the phone up by "home hey" to only see for example what time is without unlocking it, it took 1 min to turn off the screen again.
By monitoring CPU using both Bettery battery stat and CPU spy reborn I could see that most of the time CPU was occupied in 300 Mhz and it won't go to deep sleep as in JB 4.3.
Better battery stat didn't let me to find out which app is culprit as I didn't have root permission.
I tried to reboot to recovery and performed wipe cash and data/factory reset but no success; still with no app installed I got huge wakelock.
So I decided to flash the firmware one more time but tried to make a clean flash; here is procedure:
1. Delete a folder called android in internal card and move all content to PC
2. Reboot to recovery wipe cash, wipe data/factory reset(I guess this step is the key which I missed last time right before flashing the firmware)
3. Fire up odin and select what you need to flash
4. once again wipe cash, wipe data/factory reset before booting to system
4. After the procedure done install any app to monitor your CPU status to see what is going on.
That's all and I got ~90% deep sleep as I expected. Also the problem of turning off the screen(without unlocking it) is solved and it takes 5 sec to turn off now.