Information about how to measure the speed of your game are on
https://castle-engine.io/manual_optimization.php , see the section "Watch Frames Per Second".
If you actually don't do anything (or just don't do something every frame), you can set
Window.AutoRedisplay := false
to experience less usage of all resources.
By default, we have
Window.AutoRedisplay = true
, because it's easier (otherwise, when you do some low-level changes, you need to call
Window.Invalidate explicitly). And in 99% of real use-cases you have something constantly animated on the screen (maybe UI, maybe 3D etc.).
Bottom line is: CGE is optimized to display something non-trivial, maybe animated. Load something non-trivial in your scene, and check that FPS stay high
The FPS (as measured by CGE) is the number you want to watch (if you're concerned about speed of you game). And CPU usage is the number you want to watch if you're concerned about your battery life