When a CPU is going from low load state to high load state it is capable extreme temperate spikes. Sometimes the spikes could be 20+ degrees.
The application is reading the information from the Linux's kernel itself. The CPU/GPU meter application cannot read any information that kernel does not provide. Each newer android version is adding more and more restrictions on what kernel information is available. On top of that each device manufacturers are adding even more restrictions. The GPU itself is one of the most restricted components and that's why it is supported on very limited number of devices.
Starting with version 5.1.2 the application has an option to hide the notification from the lockscreen. This option is quite vendor dependable and in most cases you should enable PIN or PASS or PATTER or any other secure lockscreen in order to be able to hide the notifications from the lockscreen.
According to Android battery monitoring the app consumes 0% (which should be read as <1%) of the battery since last full change. This is achieved mainly because of the fact that monitoring stops when the screen is off.
Some devices are stopping apps more aggressively than others. That's why if you experiencing a lost of the notification then it is highly recommended to enable both "Autostart" in application settings (if applicable) and to lock the application from being closed in recent screen. At least one of two options is recommended to keep notification running. First option could be changed using the button below. Second option could be changed by opening recent screen (in most cases this screen could be opened by pressing the button right to home), long press the current application windows and then choose the "lock" icon