Every other month I play a game of Age of Empires and recently I found that it had started to lag. It would start out fine but 10 minutes into the game it would start to lag, this would get worse and worse every minute and eventually become unplayable. Even simple actions such as moving across the map became sluggish after 15 minutes of play.
Fortunately, I discovered a surprisingly simple fix: I unplugged one of my screens. Interestingly, I had used this same solution for Conquest Frontier Wars a month ago, although that game would not even start. It is worth noting that I use a USB-C cable for one of my screens, and I am playing the 2013 HD edition of Age of Empires 2. I am unsure if any of these factors are contributing to the problem.
I hope this helps someone out there :) You can maybe dig a little bit deeper, there must be an easier fix than physically unplugging one of your screens.