I have encounted some performance issues when running this DLL. The problem isn't your code, it's the way the Firaxis code deals with culture. What happens is that in a large war where you capture several cities the culture gets all mixed up (good thing) from the fighting, when you have more than two civs fighting in an area it gets even more mixed up (also a good thing). However, somewhere Firaxis has to be iterating over every tile to look at culture so what happens is that the game starts to really slow down. The slowdown will disappear as soon as you eliminate the opposing civ and their influence is removed from every tile.
One thing you could do is change it so that the cultural influence only affects tiles where you already have influence. This would preserve most of the effect your mod has on borders. Or, if you have no influence in the tile but you have > 50% in an adjacent tile it could still work. And, of course, cities would need to be an exception, perhaps forts as well. Alternatively, you could iterate over ever tile yourself and decay influence in tiles not adjacent to those where you have > 50% influence. That wouldn't speed things up during the war but it would allow things to return to normal after the war without either side being eliminated entirely.