i just got a new s9 and after leaving the phone for 5 min the power button takes 10sec delay to wake and the problem disappears after restarting the phone , i disabled every thing connected to power button such as camera launch also i choose the immediate open and close feature. the weerd thing the phone working fine but only the delay is happinig on the waking the phone
Maybe power saving mode? Check what apps are put to sleep, see if anything sounds like it should be awake.
You could try turning off any power saving and see if the phone is better, then you'll know if it's that or not.
Or an app doing stuff in the background.