You are both running the game full screen right? The windowed version is known to slow the game down.
Why the game starts in an application window is anyone's guess - probably because there is no obvious way to exit the game properly (cmd-tab-q to exit, okay, but I never use that often to exit applications) in full screen mode - ridiculous lazy programming. They have had since March to add this basic functionality. There is no internal review by Farsight of what it's like to actually use the software? The game gets ported from iOS then rushed out for sale.
Having a simple 'exit' game button on the main game UI is more user friendly than an quitting via an obscure keyboard shortcut combo.