Has anyone experienced a situation where talking to a girl in person via texting (or socials) is wildly different?

Not sure how to exactly word my question but I've been on a couple of dates with this girl that I'm interested in. Our first date was literally 5 hours long at a Bistro pub and the conversations were amazing (everything flowed naturally and there were no awkward pauses or anything like that). Second date was similar in the sense that it went longer than expected and the Nicolas Sparks were flying. However when it comes to messaging her on Instagram, she takes forever to respond (like 10 hours or more). Am I overthinking this?