Android Lollipop had memory leak issue in 5.0.x versions which has got resolved in 5.1 Android Lollipop.
Xperia Z Ultra users can now enjoy the early builds of UNOFFICIAL CyanogenMod 12.1 based on Android Lollipop v5.1.
Installing custom ROM requires a custom recovery to be installed on your device, which may VOID your warranty and may delete all your data.
Before proceeding further you may consider backup your all data.
Please visit the Disclaimer page below to agree with the risks involved and proceed before flashing this ROM to your phone:
ROM Disclaimer link
Files needed for the installation:
Installing CyanogenMod 12.1 ROM:
Copy the ROM and GApps zip file to your phone(By using a USB cable or via adb push command).
Enter into recovery mode by pressing your device specific key combinations.
Or else if you are on a pc, then type the following command:
$ adb reboot recovery
Wipe system partition, cache, data and dalvik cache (Only if you are coming from KitKat)
Select the Install Zip from SDcard option.
Select the CyanogenMod 12.1 ROM file, let the installation complete and then reboot.
Complete the initial setup, reboot to recovery and then flash the GApps package and reboot again to enjoy Android Lollipop with CyanogenMod 12.1.
Stay tuned to get more updates