It's definitely the Nvidia driver as the nail in the coffin, which is crazy to me, because it all plays acceptably fine at all max settings outside of Winters March.
Also, the Nvidia drivers with Macs are administered through OS X updates. So... it's not like they didn't know about it or it was a surprise from Nvidia. I mean, they have to know about it, because... well... it works outside of Winters March.
How the hell did they screw that up?
And yes, 14 fps during battles with the Nvidia card in Winters March, dipping down much lower with spells at times. Walking around it's ~17-23 fps. I think you were misunderstanding that 14 fps as the numbers for the Intel Iris pro.
With the Intel Iris pro I can get ~30 fps with the graphics slider all the way down, sometimes higher. It dips down to ~21-25 fps during battles, occasionally going down further into the teens.
Fortunately battles are played in "slow motion" and paused, so a terrible frame rate doesn't affect PoE as much as it would in a straight up action game.
Regardless, Winters March is pretty much running at 50% of the rest of the games performance, which is pretty terrible. Actually, I guess maybe worse, since I'm basing those numbers off of a comparison of max graphics setting to the lowest setting. Yikes..
Cheers