Three hours, no reply. You check whether the message was delivered, then whether they've been online. By the time they answer, you've written a whole story about why they didn't.
Most slow replies mean someone was busy, tired, or didn't know what to say yet. The story you write in the gap says more about your fears than about their feelings.
Read patterns, not single replies
One slow reply means nothing. A steady pattern of disappearing after closeness, or only answering late at night, tells you something. Judge someone on weeks, not on an afternoon.
A reply time is data. The meaning you give it is a guess.
If texting rhythm matters to you, say so early and lightly. Someone who wants to know you will meet you halfway.