Slow performance in later stages of game since "Trade & Prestige" update

Since updating to this new version, performance in the single player custom game is woeful in the later stages in terms of jerkiness and screen lag and everything just running slow. The game starts off fine, but towards the end when I've got a fair few regions under my belt is always starts to slow down. I have even tried a medium map with one player - same issue. Devs, please fix this, it didn't happen to me on the previous version. The game is unplayable now because it is so hard to enjoy the later stages of a game. Am running on WIndows 10, run with OpenGL.