Two ideas to address late-game slowdowns
1. Limit the total number of mines that can be built by all players collectively in the game
Once this limit is reach NO MORE mines can be built and players must capture other player's mines if they want additional mines.
2. Have the cost of mines escalate on the basis of total mines built in the game by all players rather than escalate individually for each player.
Player A builds the first mine in the game—50 drillers. Player B builds the second mine in the game—100 drillers. And so on...Perhaps the increments would need to be lessened but the idea is to make late-game mine building very expensive.
The intent is to avoid the late-game rush to build mines at the expense of military engagements that I am seeing in all my games.