Standing is timed to the gates, not to the show

Standing tickets have no seat number. They have an entry number, and people go in in that order and pick where to stand.
So Korean fans plan around when the gates open, not when the show starts. If you want the front, you arrive three to five hours before the gates.
If your number is late, arriving early changes nothing. In that case fans spend the time in the merch queue or nearby and go in when their number is called.
Eat and use the bathroom while queueing. Once entry starts you cannot leave your place.
None of this applies to a numbered seat. The number holds the place, so you can arrive for the show time.
Gate times and waiting areas differ by venue. This site lists them on each venue page along with how to get there.