Can this relationship continue? Everything is fine. I just found out that my girlfriend didn't reply to my message in time, and the response was slow.

There should be no problem according to the details you said. You're not afraid to look at his cell phone. His wechat is not afraid of you reading it. You two are together. Passion still exists. That's no problem. Just reply to the message slowly. Maybe he has a rough personality. You can talk to him and ask him to reply your message in time.