Well, as you're describing it, I see this with my son, too, and I think it *is* the processing speed thing. But I don't think it's related in any way to ADD, in my totally uninformed layperson's opinion, worth about half what you paid for it. (You have been warned there!)
What I see is that my DS just transitions more slowly, so once gets on another train of thought, he tends to stay there, especially if it's a more interesting train for him than the original.
In fact, it almost seems to me to be the opposite of ADD--thus my laser comment. It's not like he's flitting fast from one thing to another, it's more like he has a hard time moving from one thing to another, so once he goes, he's gone and he stays there.
Does that sound like what you're seeing, or is this totally different?
If I'm seeing what you're seeing, then I do think this is one perfectly normal path of development for the age of your son. It's not HG+ processing; it's ND. But it *is* normal!
And FWIW, I already see signs that DS7 is outgrowing it somewhat. He's still not lightning-quick like some HG+ kids are (and I doubt he ever will be), but he definitely does better with getting back on task when necessary without needing so much time to transition.
I guess I think he's just an ND kid in that particular area, and he's developing normally, and that's fine. <shrug> But I am no expert.
