Someone who is much more well traveled than I can probably address this, but it seems more of a european custom to set the fork on the plate with the tangs pointed down to indicate that you are still eating and placing the fork with the tangs up to show that you are finished and would like your place setting removed.
And while you can spend more time at a fastfood restaurant with little or no pressure to move on I find most are not that comfortable, inviting or intimate – with very few exceptions.
Finally, I do believe that most restaurants are geared toward “turning” the tables as quickly as possible.
