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Color me obvious, but the problem of turns taking too long can be avoided by not playing on Faerun, or any map bigger than about 200 provinces, even with 17 players in the game. When the game gets down to 4 or 5 people (or as often as not - STARTS with that few), and you're playing on Faerun, and you've got 100 provinces to deal with, that's just not going to be fun. Less fun if you aren't winning.

It's easy to pick a number of provinces per empire that you happen to like, and then assume that it scales linearly with the number of players in the game, but it doesn't. More players does mean a bigger map is required, but it's logarithmic, not linear.