Well, in my experience (given that I have a child who has been radically accelerated several times over and is pretty asynchronous with some academic skills-- and is now in college as a young-ish teen); this
is something to worry over, but probably not a reason not to accelerate in the here and now, assuming that you're talking about a jump from a primary grade (4 or 5, right?) into a higher primary grade, or even into a middle grade.
The reason is that the shift in processing speed demand would be relatively small, and probably COULD be readily accommodated, assuming that there aren't huge accommodations happening for it in the current placement.
It
does begin to matter a great deal with the work output demands are higher, and they take a VERY large jump in 9th grade, in most instances. You'd have a couple of years to figure that out or hot-house the skills, though-- but that assumes that it would be effective to do so.
It also depends upon whether it's a hard limit imposed by his particular developmental profile, or if it's voluntary in any sense. If it isn't voluntary, the problem becomes the fact that while he can be scaffolded to some degree (being given scribing, etc, keyboarding as an accommodation, or extra time on assessments), if the skill isn't truly a DEFICIT relative to the norms of academic peers, this is going to very likely come to an end in post-secondary anyway...
and, (more ominously), the ultimate problem is that it takes most high-performing/high-potential students a fair amount of time to perform at that high level. So while giving a college student time-and-a-half on a midterm exam is fine and dandy, there is not any WAY to give that same student "time-and-a-half" in the term, nor in each day. They'll come up short on having enough time to demonstrate what their peers can, in terms of mastery via work-product, first of all, and secondly, if it's truly an underlying processing speed problem, they may well need more time with the material in order to gain mastery to begin with.
Pacing of coursework is a very rude awakening for a lot of students in college, and never is that more true than with students who have slow reading or processing.
With that said, I would caution you about thinking that an acceleration is without cost down the road-- however, it might still be the right thing NOW. You can worry about a gap year or deceleration via a year abroad or something-- later.
Hope that helps.
