I always assumed slow to speech meant that he wasn't a brilliant orator and he knew it.
I think the reason God makes us blind or deaf is because it's the best thing in the long run. Maybe it brings us closer to Him, or it teaches us, or it accomplishes something indirectly. Kind of the "every gray cloud has a silver lining" idea, but the silver lining is much more important in God's plan than the gray cloud.
While I usually hold to a free-will theology, a lot of places in the Old Testament seem to contradict that. God certainly has the power to make a man react a certain way. I don't know how often he does it, various people disagree on that...
God bless you,
Grace