Do slow replies mean he's not interested?

A slow reply means something different depending on the rhythm you had before it. If he normally answers within the hour and suddenly takes a day, that change is worth paying attention to. If he has always been slow and the conversation has still felt warm, one slow reply tells you almost nothing. Look at the last few exchanges, not just the silence. Did he start the conversations, or did you? Did his messages get shorter over time, or did they stay consistent?

Also look at what he said last. If he left a question hanging, a slow reply is ordinary. If he ended with a flat "haha yeah" and nothing to pick up, the gap may be him not knowing what to say next, which is a different problem from not wanting to say anything. Context like this tells you more than the clock does.

When you do reply after a gap, the job is to give him something simple to respond to. A message that asks a light question, refers to something specific from earlier, or shares a small piece of your day works well. Keep it short enough that answering costs him almost nothing. The more effort your message asks for, the more likely it sits unanswered.

Avoid messages that carry a lot of weight: anything asking him to explain himself, anything about the silence itself, anything that reads as a test. A message that is warm, specific and asks nothing big gives him an easy way back in. It also lets you see what he does with a low-pressure opening, which is more useful information than a confrontation would give you.

A reasonable pattern is one follow-up, then a real wait. If your first message after a gap gets no reply within a day or two, send nothing else for at least a few days. Then, if you want to, send one lighter message with a different angle. If that also goes unanswered, the conversation has probably ended, even if he never says so. Two unanswered messages in a row is a clear enough signal to stop.

When he does come back after a long silence, respond warmly but do not overcorrect by suddenly matching his new pace or asking why he disappeared. Pick up the thread where it makes sense, and see whether the rhythm settles. If it does, you have your answer. If the replies stay sparse and thin, let the conversation stay small and do not put more into it than he does.