It's delayed realtime to allow people to respond if they're not logged in when attacked. If someone can hit you every 5 minutes, then if you go to bed an hour before they attack you, you're going to be dead long before you wake up.
Well yeah, But would you wake up and find your enemy sitting with a cup of coffee and a timer coming down from 4 hours, or would you wake up and find the axe flying at you again?
If they are sufficiently weakened or otherwise low on HP, and your weapon does sufficiently high damage, yes it would be possible to kill them in one hit.