To further what I said earlier, because it seems to have weight (it's nice to have people actually read and think about your posts!

)...
I used to play Civ4 on Marathon. I liked playing on that speed because I liked having more time to relax and enjoy the themes of the game. You know, that great feeling of 'building a civilization'. Civ4's gameplay was...well, meh. Stacks of death are stupid, hexes are SO much better, the civic system had neither depth nor interesting decisions (it was always obvious what to do, and there was nearly no penalty for changing your mind), and diplomacy was all about religion.
Was it a good representation of history? Sure. Was it fun? Absolutely. Was it a game? Hmmm.
Civ5, on the other hand, is certainly about the gameplay. Culture has long-term planning, no more stacks of doom, ranged attacks, both units can survive combat, upkeep costs for buildings (don't build the ones you don't truly need), less superfluous techs, hexes(!), flanking, etc. I could go on and on.
Is the flavor still there? Yeah. Is that why I play this game? No. I play it for the interesting decisions. These decisions are balanced for Standard speed. And as someone else said, the game's pacing just feels right on Standard.
Somebody else said that MP is played on Quick. I know this to be mostly true, and I find it sad. A game of Standard can take 3 hours. I know I've completed games in that time. And I've completed MP games of standard in 30 minutes. Granted, I won on turn 67...
On the other hand, I have completed MP games that lasted 10 hours. Straight. So maybe you want to play on Quick. Personally, I like Standard and the game is balanced for it. So playing a MP game on any other speed would be a waste of time. All the decisions are skewed! In ways I might not realize.