Apple iMessage SMS problem, I sent a short message, the progress bar went slowly, stuck there, and suddenly succeeded at some moment?

Apple's iMessage message progress bar went very slowly and suddenly succeeded at a certain moment. This is a unique phenomenon of Apple's mobile phone. The color of messages sent between Apple phones is different from that sent by Apple phones to Android system. There are two situations in which the mail shows that it has been sent but not read. One is that you are really hacked by the other party (the phone can be connected, but the other party can't receive the ringtone. Apple's personal mobile phone has tried it before, but the current Apple mobile phone is not very clear). Another reason is personal habits.