Does light being faster than sound have any bearing on people appearing bright until they speak?
Don’t know. None of us know what you look like. 
Fortunately, the internet travels faster than both, and we can tell when new posters aren’t that bright even before they type.
You’ve blinded me with science.
-Abraham Lincoln
Bad first post.
This being GQ, I feel compelled to offer a serious answer: Sound can travel over typical conversational distances in a time far shorter than human reaction time, so over such scales, light and sound can both be regarded as practically instantaneous. Further, while there are certainly some folks who appear bright until they begin speaking, there are also folks for whom the reverse is true, a fact which cannot be accounted for by differences in propagation speed at any distance.
Had to start with something, didn’t I?
Since the OP isn’t really asking a General Question, I’m required to move this.
Hmmmm? Where? Seems like the Pit will accommodate all comers.
samclem General Questions Moderator.
Now that’s just genius
You’d know.
A-huh-huh-huh-huh-no.
Actually, I should have said “I’m prescient.”
Wait … I thought you were Spartacus. 
Why can’t I be both?
I thought the OP’s question was mildly clever. It’s as least as good as “why do our noses run and our feet smell?” though I admit that’s not a major hurdle.
Like the late Earl Warren?
I dunno, wouldn’t a prescient Spartacus have come to a better end?
I wonder if this is the year when Jerry’s kids will finally be cured?
-Abraham Lincoln
Hmmm, I wonder what that means. Uh oh, I’d better say something…
Takes one to know one!
Zing!
/Homer Simpson
This thread is pure comedy gold, you guys! Reciting all those movie and TV references – it’s almost as funny as the clever and provocative question that started the thread off.