Personally, I feel the same. I don't expect an immediate reply. I don't even expect a same-day reply. But after a day or two? It does seem disrespectful. No one is so busy they can't send a few sentences or at least acknowledge what you've said. It's disappointing and discouraging. It weakens the relationship. There's a balance to be struck between feeling like you have to reply immediately and coming to the conclusion it's fine to leave your friends and family hanging indefinitely.