Game lags on a larger screen?

I used to play AtWar on my laptop. The game ran fine and I could always play for hours.

I recently moved to my gaming PC, which has a screen resolution about 3x the size of the one on my laptop, and for some reason, every time I zoom in to a reasonable extent, the game is choppy as hell and lags. It's almost unplayable because using the mouse to move the camera causes the game to lag. Does anyone know a solution for this? I can't go back to my laptop, so that is no longer an option.