I was always fast like that with writing. When i was writing professionally, one of my articles that earned a top award was written in about ten minutes. Can't say the editor thought the writing was less valuable because it didn't take me the full week allotted to turn it in.
I do get what you're saying. I'm a very fast reader and writer, and sometimes I get excess credit (IMO) for something that didn't take much effort. However, something great produced with little effort is generally going to get more accolades than something not as great produced with similar effort. It's not entirely fair, and someone who knows the kid well should probably take this into account--I try to pay attention to this with my kid, who has natural art talent--but it does seem to be reality. In a way, when you think about it, this discrepancy may well be part of why some people resent giftedness.