Does the game speed affect score? If I decide to go for a peaceful diplo win would it be any different score wise playing normal speed rather than epic or marathon? Or a culture game?

So far I don't see a reason to play epic or marathon if I plan on doing the peaceful route. Is there a reason?

There isn't a score modifier for game speed. However the earlier you finish the higher your score will be. Civ only takes into account the year, not the game speed, when calculating score. So playing Marathon gives you more turns before a specific year.

I believe I wasn't clear enough, so let me try again. I'm not asking if there is a game mechanic that affects score, but whether one speed is better than another when attempting to milk. Does that make more sense? Probably not.

Let's try this: If I want to go for a fast diplo or culture win, plus get as high a score as possible, strategically which speed would be the best for this goal? Is milking more effective on a slower game speed or faster one? Or is there no strategic benefit for a more peaceful game?

I hope that makes a little more sense. Thanks though Ben for replying.

Slower game speeds will always result in higher scores. Slower games mean more turns. There is no way to compensate for the turn difference for any given year.

The only difference for game speeds in relation to score is the finish year. Therefore it is more advantageous to play at the lower game speeds to give you a faster finish.

A 1000 AD diplo finish on quick would give you the same score as a 1200 AD finish on marathon if everything else was the same. However marathon will always give you a higher score. 1000 AD on quick is around 90 turns or so, on marathon, 500 turns. That means you had 410 turns to run up the score. That means 410 turns of population growth, gold generation, and cultural/land expansion. So a 1000 AD finish on Marathon will always result in a higher score.

Most game mechanics scale by game speed, but not always 1:1, and unit movement doesn't scale at all. While slower speeds give more turns to accumulate Food, Production, and Commerce, they also require a (somewhat) proportional investment. For instance, the amount of Food necessary to grow a population point is more on slower speeds, and you have more time to produce the Food. Which speed is favored by Food:Growth depends on which one has the most favorable ratio.

In general the ratios favor slower speeds. The unit movement and unit cost are the main culprits.

In some cases though, faster speeds can be favored (by date, not sure if it holds up for score), like Cultural comparing Quick and Normal favors Quick. (I haven't checked these numbers since release.) That is because the number of turns is 330:440 (3:4), but the Culture requirement for the cities is 25k:50k (1:2), meaning you have relatively more Turns:Culture on Quick (3:1) than Normal (4:2). The unit-based advantages for Normal over Quick don't apply so much to the Cultural date (but certainly can by score), and the other significant imbalance that favors Normal, the Great Work, can be minimized by using GP settlement more or going with an approach that focuses more on Wonders and Commerce than Artists.

If you check the current HOF all of the fastest finishes are at the slower speeds. However there is an Epic game that has a higher score than a Marathon game that finished earlier, and there is a quick game that has a higher score than a normal game that finished earlier. Those were both cultural, I didn't check the other conditions, and those were the only exceptions.