– Under “Display” in Settings, set screen to go into sleep mode after 30seconds or less (depends on user); And/Or;
– Disable Bravia Screen Optimiser and Auto Lighting adjustment under “Screen Brightness” in Settings; and/or,
– In Developer Options, you could adjust screen rendering to require less processor power.
Much newer models like the Z5, Z4v, etc can actually activate the “Stamina Mode” option under settings. This limits the functions of the phone to just basics, as well as connections to the Internet for browsing or GPS location.