The steps to use it:

1. import the ste into your current project
2. drag and drop FPS_Display_Main AI into the Game/MainUserAi window
3. drag and drop FPS_Display HUD into Game/Resources
4. open FPS_Display_Main/onInit to customize the script

Why the counter never really goes bellow 10 frames per sec ?

The frame rate is clamped, because the maxFrameTime is set to 0.1 by default.
You can change this value, for example:

application.setMaxFrameTime (  0.5 ) //FPS > 2