Are there any modifications to settings than can shorten how long it takes to play the game?

Change the boot time. When creating a game > Change Boot/Surrender

There is a way to shorten a game in terms of actual time, like Dan said, but AFAIK it's not possible to make it last less turns. Different kinds of maps and settings will make the game tend to last shorter/longer, but basically the game ends when it ends - when there's only one player remaining (other players are eliminated, surrendered or booted), or when it is voted to end.

(if it's what you're looking for)

There's a lot of game settings that can certainly prolong the game such as Diplo cards, Reinf cards, high value bonuses, low kill ratios, huge maps, etc.

If you choose a small map, no cards, regular kill ratios, etc - you can absolutely have a short game in # of turns.