In our experience, slow production speeds can cause problems with teachers in school, perhaps because of stereotypes about GT kids. If a teacher thinks that all GT kids produce results fast, a slow writer may be seen as "not anything special." Some teachers miss that a child may take in info very fast and yet may not write his answers at all quickly.
Slow production can be exacerbated by boring work--it was for my DS7. And that often makes the "He's not that smart" thing even worse, since the teacher figures that if he's THAT slow on the normal work, there's no way he should be doing anything harder. It becomes a reason not to differentiate the work.

To me, if the slow production isn't happening because writing hurts the child's hand or wears him out, then it isn't anything to get too worried about until/unless it causes problems. But I think it pays to be aware of what problems you might run into...