I assume you mean the 40% cpu is being used by kodi.bin ?
Does it drop if you reboot or does it go straight back to 40% ?
The two things in Kodi that are likely to use CPU when “idle” are addons performing activities in the background or Skin animations. Is there anything animating/moving on the screen, even scrolling text ? If so, 40% would be typical, and you might see about 20% with nothing on the screen moving.