Thanks for the comments, guys!
The reason it decreases hosting time for turns is directly related to the number of units that have been recruited in the game. Each one of those units takes up memory, requires appropriate calculations in battle etc, which slows the game down. That's why the hard limit for units that can be on the map at the same time was only tripled from Dom2 instead of being maxed out. Maxed out, it would bring any system, no matter how top of the line, to its knees.