I play this game almost every day with a 13-inch Retina MacBook Pro with an i5 and only the Iris graphics and I can play literally 14 straight hours without any real issues. (I get bored on weekends) My machine has an SSD, which might be important, especially if the game is hitting virtual memory a lot, but otherwise my system is basically identical to yours except that it's got slightly worse hardware across the board.

Report your performance issues in the bug forum for the mac beta, but also consider trying to do general system health stuff like verify and repair disk permissions, restart your system, and check that you aren't running a lot of other software and monitor the performance with Activity Monitor.

My best guess is that you've got an unfortunate memory leak that's causing you to hit virtual memory a lot more heavily than you should be.

You should be able to see this in activity monitor, but also by checking your "available disk space" while it's acting up and then shortly after you shut it down. If it's eating virtual memory then you should see a fairly dramatic change in your available hard drive space when you shut the game down and it frees up the block of virtual memory.

The more details you can give the devs about what exactly is happening and what the symptoms are the more likely they are to be able to track down and fix it. The game isn't generally unplayable on a newer mac (so I wouldn't take the advice of the people encouraging you to install windows just yet). It's not well-optimized, but it's more than satisfactory, you've just got a bug a lot of us aren't seeing, or aren't suffering from as severely. Helping the devs diagnose it and getting it fixed could mean a better experience for everyone. Good luck!