I don't think its particularly slack to not respond straight away either.... I work and leave my phone in the back office. I can check it once a day, so I won't be able to respond. Sometimes I miss texts all together.

I guess what I'm saying is, don't assume someone is being slack or rude because they don't respond. Especially to a text, if it truely is that important, ring. I hear my phone ring, and make sure I check to see who phoned and try to get back with them, I miss texts....