Hi!
Usually there are 6 working cores out of 8. This is the cause of bad energy saving. You could using Kernel Adiutor or editing file /sys/devices/system/cpu/cpu0/core_ctl/min_cpus.
But on version 8.1, the value is restored after the change, so it must be committed.
You must execute this simple script after each reboot. (You can move it in init.d dir) Dont forget give execution rights to script
ex. chmod 777
Code:
#!/system/bin/sh
echo 0 >> /sys/devices/system/cpu/cpu0/core_ctl/min_cpus
chmod 555 /sys/devices/system/cpu/cpu0/core_ctl/min_cpus