Messages Take Long Time to Send

On my iPod Touch 5th generation, when I send a message to another person that's on the same wifi network it takes 2 - 10 minutes to get to them. So it says it's sent and then it takes 2 - 10 minutes for it to actually get to them. That is if me and the other person are on the same wifi network. Yet if we aren't on the same network (most of the time) it gets to them instantly. Why does this happen? I know it's strange to send a message to someone who's in the same area and not just talk to them. I usually do this if I'm at a party with my family and I don't want to talk about something with them in front of everyone else.