Don't deliberately delay your replies to seem more interesting or busy - that's an outdated tactic that just creates uncertainty. If you read a message and think "I'll respond later," that often comes across as indifference, especially early on when someone isn't sure about your feelings yet. They're responding quickly because they're interested, and a delay can demotivate them.
But replying a few hours later is totally fine if it happens naturally. People have lives, they're busy with work, school, stuff. Nobody expects an instant reply to every message. It's different if you see it 10 minutes after they send it but don't respond until the next day. That can genuinely look like you're playing it cool.
Go by the overall vibe of the conversation, not the clock. If the chat is flowing and interesting, reply within an hour or two when you get a chance. If you two already have that understanding, then waiting a few hours won't be a problem. The main thing is making sure your replies are thoughtful and show you're actually reading what they're saying, not just ticking a box. After a first date, it's better to show interest through how quickly and thoughtfully you respond rather than playing games with timing.