Delay after pressing button, kind of OH hibernation?

I detected strange behaviour of OpenHab. I have Rpi (when I installed OH) and MCP23017, all control buttons are connected to RPi through MCP.
When user does not press any button and not trigger any rules for longer time OpenHab (or Rpi) switches to something I can name sleep mode.
Simply when user presses button OH reacts in 2-4 seconds, it means with long delay after that it reacts very fast. But when I wait 15-20 and press button again, situation happens again. I have feeling that OH or Rpi switch to kind of hibernation.