Are there not varietals within what we call "english" walnut bred to grow faster to reach nut-producing age sooner? I believe I've heard this from several people that at least seem to know what they are talking about. Macintosh and granny Smith are both "Apple", I'm lead to believe Walnut is similar.
I'd think local climate, soil type, amount of moisture, etc would have a profound effect, but also if there truly are varietals then that may also have some noticable effect. Here's at least one link detailing some of the varieties...I have heard that Franquette makes good stock wood, and have heard of some of the others, but don't really know much about them. I believe most are Juglans Regia though.
walnut cultivars 
