As someone who (I think it's probably fair to say) doesn't have ADHD (but possibly has something else diagnosable, though hasn't sought a diagnosis), I'll at least throw in my 2¢, just to explain why I don't usually immediately answer texts.

It's kindof... physically painful?... to know something's wanted from or expected of me. Social interaction (face-to-face, telephone, text, email, whatever) takes effort. I'd say I manage it quite well, but only by spending a lot of my time alone and completely not thinking about other people who I might possibly ever interact with. It's how I "recharge."

I don't often get or send texts. But if I get a text while in "not thinking about people" mode, that gives me a lot of anxiety. If I get a text while I'm in "adulting and socializing" mode (like while I'm working or out shopping or even just in the presence of or in a conversation with humans in any capacity), the text is the thing that always sinks to the bottom of my priority queue.

So, if I answer a text, it's probably going to be at the next point at which I'd otherwise be shifting contexts out of "socializing" mode and into "alone" mode. The end of the workday for instance.

That said, I basically never initiate text conversations. If I have a text conversation, it's someone else who intiated the conversation. So perhaps this doesn't quite fit your question. But also, as soon as I do respond, I remain responsive until the conversation ends.

Also, I'm a bit of a Luddite in general. So that might also have something to do with it.