I'm just going by what I think - no research or other hard info.
Though I did see a web page somewhere recently that recommended filling slowly. It also recommended filling in the a.m. as that's when the gas in storage tank is coldest, thus densest. If you buy in late afternoon / early evening you're buying warmed "expanded" gasoline but paying the same price per gallon.
I figure at a fast rate there's lots of splashing. I'd guess the fuel backing up the pipe makes the handle click off sooner. Also maybe a bit more randomly for different fillups, so your consistency might go down. And I'm guessing that the splashing makes more vapors that get sucked out by the vapor return line that most pump hoses have now.
well, i know what i'll be testing next. thanks! good info.