people who expect chat to be real-time are usually people who hate to be kept waiting and who immerse themselves in the other person fully for a while. like you would in a face-to-face conversation.

the other type of person uses chat more like a messaging service. writing telegrams instead of being present for the conversation.

Snapchat is very much designed around being present and talking in real-time. when the other person checks the chat window, you get notified, so you know they're there. it resembles real-life interaction a bit more.